Module 1 – Development of practical skills in chemistry

Question 1

Explain the method of PAG5- Preparation of a pure organic liquid:

Purifying an organic liquid

Answer 1

General method:
• Put the distillate of impure product into a separating funnel

• wash product by adding either: sodium hydrogencarbonate solution , shaking and
releasing the pressure from CO2 produced or sodium chloride solution

•Allow the layers to separate in the funnel by shaking , and then run and discard the aqueous layer.

•Run the organic layer into a clean, dry conical flask and add three spatula loads of drying agent (e.g. anhydrous sodium sulphate,calcium chloride) to dry the organic liquid. During this stopper the flask and shake for a few seconds then allow the solid to settle. When dry the organic liquid should appear clear.

• Carefully decant the liquid into the distillation flask

•Distill to collect pure product

Key points:

• Sodium hydrogencarbonate will neutralise any remaining reactant acid.

• Sodium chloride will help separate the organic layer from the aqueous layer

• The layer with lower density will be the upper layer. This is usually the organic layer

• The drying agent should be insoluble in the organic liquid and not react with the organic liquid

• Decant means carefully pour off organic liquid leaving the drying agent in the conical flask

Question 2

Explain the method of distillation

Answer 2

In general used as separation technique to separate an organic product from its reacting mixture. Need to collect the distillate of the approximate boiling point range of the desired liquid.

Note the bulb of the thermometer should be at the T junction connecting to the condenser to measure the correct boiling point

Note the water goes in the bottom of the condenser to go against gravity. This allows more efficient cooling and prevents back flow of water.

Electric heaters are often used to heat organic chemicals. This is because organic chemicals are normally highly flammable and could set on fire with a naked flame.

Question 3

Explain the method for reflux

Answer 3

Reflux is used when heating organic reaction mixtures for long periods. The condenser prevents organic vapours from escaping by condensing them back to liquids. The reactant vapours of volatile compound are condensed and turned to the reaction mixture.

Never seal the end of the condenser as the build up of gas pressure could cause the apparatus to explode. This is true of any apparatus where volatile liquids are heated including the distillation set up

Anti-bumping granules are added to the flask in both distillation and reflux to prevent vigorous, uneven boiling by making small bubble form instead of large bubbles

Question 4

Explain the general method for titration

Answer 4

General Method
•rinse equipment (burette with acid, pipette with alkali, conical flask with distilled water)

•pipette 25 cm3 of alkali into conical flask

•touch surface of alkali with pipette ( to ensure correct amount is added)

•adds acid solution from burette (make sure it is rinsed with the substance inside the burette)

•make sure the jet space in the burette is filled with acid and air bubbles removed

•add a few drops of indicator (as they are weak acids and could affect results) and refer to colour change at end point

•phenolphthalein [pink (alkali) to colourless (acid): end point pink colour just disappears] [used when using strong alkalis e.g when NaOH is used]

•methyl orange [yellow (alkali) to red (acid): end point orange] [use when using string acids e.g when HCl is used]

•use a white tile underneath the flask to help observe the colour change

•add acid to alkali whilst swirling the mixture and add acid drop wise at end point. Distilled water can be added to the sides of the conical flask during titration so that all acid is washed into the reaction mixture to react with the alkali.

•note burette reading before and after addition of acid. Measurement to the nearest 0.05cm^3.

•repeats titration until at least 2 concordant results are obtained- two readings within 0.1 of each other

Key ideas:

Wear PPE (acids and alkalis are corrosive and irritants )

Question 5

Explain the method for measuring mass loss in thermal decomposition

Answer 5

Decomposition reaction. The water of crystallisation in calcium sulphate crystals can be removed as water vapour by heating as shown in the following equation:

CaSO, xH,0(s) → CaSO,(s) + *H,O(g)

Method:

•Weigh an empty clean dry crucible and lid

• Add 2g of hydrated calcium sulphate to the crucible and weigh again

• Heat strongly with a Bunsen for a couple of minutes

• Allow to cool

•Weigh the crucible and contents again

• Heat crucible again and reweigh until you reach a constant mass (do this to ensure reaction is complete).

Key ideas:
The lid improves the accuracy of the experiment as it prevents loss of solid from the crucible but should be loose fitting to allow gas to escape/enter.

Large amounts of hydrated calcium sulphate, such as 50g, should not be used in this experiment as the decomposition is likely to be incomplete.

The crucible needs to be dry otherwise a wet crucible would give an inaccurate result. It would cause mass loss to be too large as the water would be lost when heating.

Small amounts of the solid , such as 0.100 g, should not be used in thisexperiment as the percentage uncertainties in weighing will be too high.

Question 6

How would you measure gas volume with a gas syringe (using an example)

Answer 6

1) Extract 0.20 cm3 of propanone into a hypodermic syringe and then measure the mass of this syringe
using hand protection, remove a gas syringe from the oven and note the volume of air already in the
barrel – about 5 cm3.

2) . inject the propanone through the self-seal cap into the barrel. The plunger will move straight away.
Put the gas syringe back into the oven.

3) Measure the mass of the empty hypodermic syringe immediately.

4) After a few minutes measure the volume of the gas in the gas syringe, record the temperature of the oven shelf and the pressure of the room.

Potential errors in using a gas syringe:

•gas escapes before bung inserted

•syringe sticks

• some gases like carbon dioxide or sulphur dioxide are soluble in water so the true amount of gas is not measured.

Question 8

Explain the method to create a simple standard solution

Answer 8

Method:
• Weigh the sample bottle containing the required mass of solid on a 2 dp balance

• Transfer to beaker

• Reweigh empty sample bottle

• Record the difference in mass

• Add 100cm3 of distilled water to the beaker. Use a glass rod to stir to help dissolve the solid.

•Sometimes the substance may not dissolve well in cold water so the beaker and its contents could be heated gently until all the solid had dissolved.

• Pour solution into a 250cm3 graduated flask via a funnel.

• Rinse beaker and funnel and add washings from the beaker and glass rod to the volumetric flask.

• make up to the mark with distilled water using a dropping pipette for last few drops.

• Invert flask several times to ensure uniform solution.

Key points:
Alternatively the known mass of solid in the weighing bottle could be transferred to beaker, washed and washings added to the beaker.

Remember to fill so the bottom of the meniscus sits on the line on the neck of the flask. With dark liquids like potassium manganate it can be difficult to see the meniscus.

Shake the volumetric flask thoroughly toensure a uniform concentration

Question 10

Explain the method in order to determine the composition by mass of copper (II) carbonate (PAG1)

Answer 10

Method:

1) set up apparatus as shown on written flash card

2) 2. Weigh 1.5 g of CuCOg.Cu(OH), and add it to the conical flask.

3. Measure 50 cm of H,SO, in a 50 cm’ measuring cylinder.
4. Add the H,SO, into the conical flask and immediately insert the stopper. The gas will collect in the measuring cylinder.
5. Record the final volume of carbon dioxide in the measuring cylinder.

Calculations:
• Calculate the volume of carbon dioxide collected

• Work out the amount of copper carbonate, CuCO, that reacted in moles

• Calculate the percentage by mass of CuCO, in the original sample of CuCOg.Cu(OH)2-

Question 11

Evaluate what can occur during the experiment to determine the composition by mass of copper (II) carbonate (PAG1)

Answer 11

Errors:

• Some carbon dioxide may escape before the bung is inserted.Insert the bug as soon as the acid has been poured into the conical flask.

Some copper carbonate may not react.
• Swirl the conical flask to mix the contents and ensure that the reaction goes to completion.

• Carbon dioxide may dissolve in the water.
• Use a gas syringe to measure the volume of CO, instead.

• Some copper carbonate may remain in the weighing boat meaning it’s not transferred to the conical flask.
- Use the weighing by difference technique (weigh the weighing boat with the copper carbonate, add the copper carbonate to the conical flask then reweigh the empty weighing boat. The difference between these masses is the exact mass of copper carbonate).

Question 12

Explain the method in order to determine the relatice atomic mass of magnesium (PAG1)

Answer 12

1. Set up the apparatus as drawn on the written flash card
2. Add 10 cm’ of sulfuric acid to the conical flask.
3. Weigh 0.15 g of magnesium. Add the magnesium to the acid, placing the bung into the flask immediately. The gas produced during the reaction will collect in the measuring cylinder.
4. Record the final volume of hydrogen in the measuring cylinder.
5. Retain the solution in conical flask for method

Method 2:

1. Weigh a clean evaporating basin.
2. Pour the solution produced during method 1 into the evaporating basin.
3. Rinse the conical flask with some distilled water and add the washings to the evaporating basin.
4. Evaporate the solution until a dry solid is left in the evaporating basin.
5. Leave to cool then reweigh the evaporating basin containing the solid magnesium sulphate.
   The mass of magnesium sulfate is the difference between the final mass and the mass of the empty basin.

Calculation:
• Calculate the amount of magnesium sulphate formed, in mol.

• Work out the amount of Mg in MgSO,

• Use the original mass of Mg calculate the relative atomic mass of magnesium

Question 13

Evaluate the method in order to determine the relative atomic mass of magnesium (PAG1)

Answer 13

Errors:
• The errors for method 1 are similar to experiment 1.1.

• Solid magnesium sulphate may not be completely dry

• To ensure complete dryness heat until the mass no longer changes.

Question 15

Explain the method in order to determination for the formula of magnesium oxide (PAG 1)

Answer 15

1.Measure the mass of the crucible and lid.

2.Put the magnesium ribbon to the crucible. You will need to coil the magnesium so that it fits. Reweigh the crucible and lid.

3.Arrange the apparatus as below and heat the crucible strongly. Raise the crucible lid slightly using tongs to control the reaction.

4.When the reaction is nearly complete, place the crucible lid on the heat-proof mat and heat the crucible strongly for 5 minutes. During this time, tap the magnesium oxide gently with tongs to break up the residue.

5.Allow the crucible to cool and reweigh the crucible, its contents and the lid.

Question 16

Draw the apparatus for PAG 1 determination of the composition of copper (II) carbonate and determination of the relative atomic mass of magnesium

Question 17

Draw the apparatus for PAG 1 the determination for the formula of magnesium oxide

Question 18

Explain the procedure of preparing a standard solution

Answer 18

1) Place the weighing boat on the balance and measure its weight. Then place the weighing boat with the solute on the balance. After transfer the solute into a 100cm^3 beaker then weigh the weighing boat again using the balance.

2) Pour 100cm^3 of distilled water with the solute in the beaker and stir with a glass rod

3) We then use a funnel and transfer he solution into a volumetric flask. We rinse off the beaker, glass rod and funnel with distilled water and include the rinsing inside the volumetric flask until the bottom of the meniscus reaches
250сm^3

4) Place a stopper above the volumetric flask and invert it several times to ensure through mixing.

Question 19

Explain the method to determine the concentration of HCl (PAG 2)

Answer 19

Method:

Part 1: Making a 0.10 mol dm^3 standard solution of NaHCO,

1. Weigh 2.10 g of sodium hydrogencarbonate into a weighing boat then transfer to a 250 cm^3 beaker.
2. Add a small volume of distilled water to the beaker and stir until the solid completely dissolves.
3. Transfer the solution to a 250 cm^3 volumetric flask using a funnel.
4. Rinse the beaker and the glass rod with distilled water, adding the washings to the volumetric flask.
5. Fill the volumetric flask up to the graduated line using distilled water.
6. Stopper the flask and invert a few times to mix the solution.

Part 2: Finding the concentration of HCI by titration:

1. Use a pipette and pipette filler to transfer 25 cm^3 of the standard NaHCO, solution to a 250 cm^3 conical flask.
2. Add two or three drops of methyl orange to the flask.
3. Fill the burette with hydrochloric acid (ensuring there is no air bubble) and record the initial burette reading
4. Add the hydrochloric acid from the burette to the conical flask until the end point is reached.
   Add the acid dropwise as you near the end point. At the end point, the solution will change from yellow to orange in colour.
5. Record the final burette reading. The volume of HCl added is the difference between the initial and final readings.
6. Repeat until two concordant results are obtained.

Calculations:
* Calculate the mean titre using the concordant results.

• Calculate the moles of NaHCO, present in 25 cm^3 of standard solution.

• Calculate the moles of HCl present in the mean titre, using the following neutralisation
equation: HCl + NaHCO3 -> NaCl +H2O + CO2

• Calculate the concentration of HCI.

Question 20

Evaluate the errors in the method to determine the concentration of HCl (PAG 2)

Answer 20

Errors:
• Some of the weighed NaHCO, may not be transferred from the weighing boat.

• Use the weighing by difference technique

• Take care not to spill any solid or solution during transfer.

• Forcing all the liquid from the pipette

• The pipette is calibrated to account for the liquid that remains in the tip.

• During the titration, swirl the conical flask when adding the acid to ensure that the reactants are well mixed.]

• The colour change may not be clearly visible during the titration

• Place a white tile underneath the conical flask.

Question 21

Explain the method for the qualitative analysis of ions (PAG 4)

Answer 21

Method:

Part 1: Identifying the negative ions (anions) present in (unknown) mixture A
1. Add 25 cm^3 of distilled water into a 100 cm^3 beaker.

2. Add mixture A into the water and stir with a glass rod, until most of the solid has dissolved.
3. Filter the mixture into a conical flask.
4. Add about 2 cm of the filtrate to a test tube.
5. Add 5 drops of HNO, followed by 10 drops of AgNO, to the test tube. Record any observations.
6. To the same test tube, add NH, dropwise until there is no further colour change. Record any observations.
7. Add 1 spatula of the residue from step 3 to a test tube. Add about 2 cm^3 of HNO, to this test tube. Continue adding HNO, dropwise until there is no further reaction. Record any observations.
8. Use your observations to suggest formulae of the two anions in mixture A and write equations for the observed reactions.

Part 2: Identifying the Group 2 positive ion (cation) present in mixture A

1. Add about 2 cm^3 of the filtrate from part 1, step 3 to a test tube. Add about 2 cm^3 of sulfuric acid to this test tube and record any observations.
2. In a new test tube, add 2 cm^3 of the filtrate from part 1, step 3 followed by about 2 cm^3 of K2CrO4. Record any observation.
3. Deduce the formula of the Group 2 positive ions in mixture A using your observations.

Question 22

Explain the method to determine the enthalpy of combustion of an alcohol (PAG 3)

Answer 22

PROCEDURE:
a) Weigh a spirit burner (with its lid), record the mass to the nearest 0.01g and note the name of the alcohol.

b) Using a 100cm^3 measuring cylinder, measure 100 cm^3 of water into a beaker.

c) Record the initial temperature of the water to the nearest 0.5 C

d) Set up the apparatus as drawn on written flash card

e) Light the burner using a lighted splint and burn the alcohol whilst stirring the water carefully with the thermometer

f) After approximately three minutes extinguish the flame. Immediately record the maximum temperature reached by the water to the nearest 0.5C.

g) Reweigh the spirit burner and record the mass to the nearest 0. 01 g

h) Repeat steps 1-7 for the different alcohols in the other spirit burners.

Question 23

Draw the apparatus to determine the enthalpy of combustion of an alcohol (PAG 3)

Question 24

Explain the method for the determination of the enthalpy change of neutralisation (PAG 3)

Answer 24

1. Measure 25 cm^3 hydrochloric acid using a 25 cm^3 measuring cylinder and add it to a polystyrene cup.
2. Place the cup in a 250 cm^3 glass beaker.
3. Construct a suitable table to record the temperature of the acid at minute intervals for up to 10 minutes
4. Measure 25 cm^3 of sodium hydroxide into a 25 cm^3 measuring cylinder.
5. Start the timer and record the initial temperature of the hydrochloric acid in the cup.
6. Continue to record the temperature each minute for three minutes.
7. At the fourth minute, add the sodium hydroxide to the cup. Do not record the temperature.
8. At the fifth minute, continue recording the temperature up until 10 minutes. Stir the solution in the cup each time the temperature is recorded.

Calculations:
* Plot a graph of temperature (y axis) against time (x axis).

• Draw a line of best fit through the points before the addition of the sodium hydroxide then draw a second line of best fit through the points after the addition of sodium hydroxide.
  Extrapolate both of these lines to 4 minutes.
• Use the graph to determine the temperature change at the fourth minute.
• Calculate the energy absorbed by the solution using the equation q = mcAT .
• Calculate the amount of HCI used.
• Calculate the enthalpy change of neutralization.

Question 25

Draw the apparatus for the determination of the enthalpy change of neutralisation (PAG 3)

Question 26

evaluate the method for the determination of the enthalpy change of neutralisation (PAG 3)

Answer 26

Errors: • Heat transfer to and from surroundings. - Inaccuracy in temperature measurement. • Electronic thermometer can be used.

Question 27

Explain the method for the qualitative analysis for identifying organic unknowns and results-alkanes (PAG 7)

Answer 27

Chemicals provided for the experiment: • Heptane • Cyclohexane • Cyclohexene - Limonene • Bromine water 1. Set up a water bath in a 250 cm^3 beaker. Leave this to one side. 2. Heat each haloalkane separately under reflux with sodium hydroxide to release the halide ions. 3. For each of the haloalkanes, in a separate test tube: a. Add five drops of the refluxed haloalkane solution. b. Add 1 cm^3 of ethanol and 1cm^3 of silver nitrate solution to the test tube. c. Shake side to side to mix the solution well, and place the test tube into the water bath. d. Record any observations after 3 minutes. Expected results: Heptane- orange Cyclohexane- Two separate layer forms, top layer orange and bottom layer water Cyclohexene- Colourless Limonene- Colourless

Question 28

Explain the method for the qualitative analysis for identifying organic unknowns and results- haloalkanes (PAG 7)

Answer 28

Chemicals provided for the experiment: • 1-chlorobutane • 1-bromobutane - 1-iodobutane • Ethanol • Aqueous silver nitrate 1. Set up a water bath in a 250 cm^3 beaker. Leave this to one side. 2. Heat each haloalkane separately under reflux with sodium hydroxide to release the halide ions. 3. For each of the haloalkanes, in a separate test tube: a. Add five drops of the refluxed haloalkane solution. b. Add 1 cm^3 of ethanol and 1cm^3 of silver nitrate solution to the test tube. c. Shake side to side to mix the solution well, and place the test tube into the water bath. d. Record any observations after 3 minutes. Expected results: 1-chlorobutane- White precipitate forms. Slow reaction. 1-bromobutane- Cream precipitate forms. Quick reaction 1-iodobutane- Yellow precipitate forms. Very quick reaction

Question 29

Explain the method for the qualitative analysis for identifying organic unknowns and results- aldehydes (PAG 7)

Answer 29

Chemicals provided for the experiment: • Fehling's reagent • Tollens' reagent • Brady's reagent • EthanalI • Propanone 1. Set up a hot water bath. 2. Add 2 mL Fehling's reagent to two separate test tubes and place in a test tube rack. 3. Add a few drops of ethanal to one test tube and a few drops of propanone to the other test tube. Warm in the water bath and record any observations. 4. Add 2 mL Tollen's reagent to two separate test tubes and place in a test tube rack. 5. Add a few drops of ethanal to one test tube and a few drops of propanone to the other test tube. Warm in the water bath and record any observations. 6. Add 2 mL Brady's reagent to two separate test tubes and place in a test tube rack. 7. Add a few drops of ethanal to one test tube and a few drops of propanone to the other test tube. Record any observations. Expected results: Ethanal: Ethanal mixed with Fehling’s reagent= brick red precipitate forms Ethanal mixed with Tollen’s reagent= silver mirror forms on the surface of the test tube Ethanal mixed with BRaady’s reagent = Orange precipitate forms Propanone: Propanone mixed with Fehling’s reagent produced no precipiate, solution remains deep blue Propanone mixed with Fehling’s reagent produces no change Propanone mixed with Brady;s reagent forms an orange precipitate

Question 30

Explain the method for the qualitative analysis for identifying organic unknowns and results- alcohols (PAG 7)

Answer 30

Chemicals provided for the experiment: • Butan-1-ol - Butan-2-ok 2-methylpropan-2-ol • Phenol • Acidified potassium dichromate solution Bromine water 1. Heat the butan-1-ol under reflux with the acidified potassium dichromate and make a note of any observations. 2. Repeat for butan-2-ol and 2-methyl-propan-2-ol. 3. Add bromine water to a sample of phenol and note any changes. Expected results Butan-1-ol forms a colour change from orange to green (primary alcohol) Butan-2-ol forms a colour change from orange to green (secondary alcohol 2-methylpropan-2-ol solution remains orange in colour (tertiary alcohol) Phenol causes a solution of bromine water to change from orange to colourless and a white precipitate will form

Question 31

Explain the method for the qualitative analysis for identifying organic unknowns and results- carboxylic acids (PAG 7)

Answer 31

Chemicals provided for the experiment: • Ethanoic acid - Dilute sodium carbonale salution - Limewater 1. Place a test tube containing limewater in a test tube rack, with the delivery tube in the solution. 2. In a second test tube, add a few mL of sodium hydrogen carbonate solution, then add a few mL of ethanoic acid and immediately insert the bung attached to the delivery tube. 3. Bubble the gaseous product from the delivery tube through the limewater. Expected Results: The ethanoic acid reacts with the sodium hydrogen carbonate, producing carbon dioxide. Effervescence will be observed. CO, turns limewater cloudy.

Question 32

Explain the method for PAG 5 the synthesis of a haloalkane

Answer 32

Part 1Preparation: 1. Pour about 6.5 cm^3 of 2-methylpropan-2-ol into a 10 cm^3 measuring cylinder then weigh the measuring cylinder and its contents. 2. Pour the 2-methylpropan-2-ol into a 50 cm^3 separating funnel. 3. Reweigh the empty measuring cylinder to deduce the mass of 2-methylpropan-2-ol used in the reaction. 4. Measure 20 cm^3 of concentrated hydrochloric acid and gradually add the acid to the separating funnel. 5. Place the stopper on the separating funnel and shake it vigorously for 20 minutes, releasing the pressure when required. Part 2 seperation: 1. Allow the mixture to separate. 2. Once separated, remove the stopper and open the tap to collect the bottom aqueous layer. 3. One all the aqueous layer has been removed, close the tap and add 10 cm^3 of 5% sodium hydrogencarbonate to the organic layer in the separating funnel. 4. Shake the mixture in the funnel gently, releasing the pressure when required. 5. Allow the mixture to separate and remove the bottom aqueous layer. 6. Repeat steps 3 to 5 until there is no pressure build up in the separating funnel. 7. Collect the organic layer into a 100 cm^3 conical flask. 8. Add anhydrous magnesium sulfate slowly while swirling the conical flask, until the magnesium sulfate stops clumping. 9. Filter the solution through a filter paper into a clean round-bottom flask. Part 3 Distillation: 1. Weigh an empty sample tube. 2. Set up the apparatus as drawn on my flash card 3. Add the organic sample from the conical flask with anti-bumping granules to the round bottom flask and connect it to the apparatus. 4. Collect the liquid impurities that come through the condenser below 48°C in a small beaker. 5. When the temperature reaches 48°C, collect the liquid from the condenser in the pre-weighed sample tube until no more liquid comes through the condenser. 6. Weigh the sample tube and calculate the mass of the product.

Question 33

Key point behind PAG 5 synthesis of a haloalkane

Answer 33

Key Points: • A water bath or electric heater should be used to heat the mixture if there are flammable substances present. • Anti-bumping granules prevent large bubbles from forming and ensure that the liquid doesn't boil too vigorously. If anti-bumping granules weren't used, the mixture would boil over into the condenser meaning impurities would contaminate the product. • The condenser should be tilted down slightly to allow any liquid to run into the collection flask. • The bulb of the thermometer should be at the T-junction connecting to the condenser to correctly measure the boiling point. • The water must enter at the lowest point and leave at the highest point of the condenser to ensure that it is filled with water. This will maximise heat transfer for condensation so that cooling is more efficient. • The collection flask must not be sealed to the condenser as this would make the system airtight. As heating occurs, air inside the system expands so air must be able to escape to avoid cracking the apparatus.

Question 34

Explain the method in order to prepare cyclohexene PAG 5

Answer 34

Part 1 Preparation: 1. Pour 10 cm^3 of cyclohexanol into a 50 cm^3 pear-shaped or round-bottomed flask 2. Using a plastic graduated dropping pipette, carefully add approximately 4.0 cm^3 of concentrated phosphoric acid to the flask. 3. Add a few anti bumping granules to the flask and then assemble the reflux apparatus as written on flash card

Question 35

Explain the method, analysis and evaluation for PAG 9 rate of reaction between magnesium and hydrochloric acid

Answer 35

Method: 1. Assemble the apparatus as written on my flash card. Add 50 cm^3 of HCI to the conical flask, then add a 6 cm strip of magnesium to the conical flask. Immediately insert the bung and start the timer. 3. Record the volume of gas at 15 second time intervals for 2.5 minutes. 4. Repeat for different concentrations of HCl Analysis: 1. For each HCI concentration: Plot a graph of time (sec.) on the X-axis against volume of Ha gas produced (cm') on Y-axis. 2. Draw a line of best fit for your time-points on graph. 3. Draw a tangent at time = 0 s. Use the gradient to find the initial rate for each concentration of HCI. Errors: Some gas may have escaped before putting the bung on.