required practicals

Question 1

What is the aim of the titration practical?

Answer 1

To determine the concentration of an acid or alkali by neutralization with a standard solution.

Question 2

What apparatus is essential for a titration?

Answer 2

Burette, pipette, conical flask, volumetric flask, indicator, and standard solution.

Question 3

How do you prepare a standard solution for titration?

Answer 3

Weigh an accurate mass of solid, dissolve it in distilled water in a volumetric flask, and make up to the mark.

Question 4

What is the purpose of a pipette in titration?

Answer 4

To measure a precise volume of solution to be titrated.

Question 5

Why do we use an indicator in titration?

Answer 5

To show the end point by changing colour at the equivalence point.

Question 6

Name a common indicator used in acid-base titrations.

Answer 6

Phenolphthalein or methyl orange.

Question 7

How do you calculate the concentration of an unknown solution after titration?

Answer 7

Use the balanced equation, volume and concentration of the known solution, and the volume of the unknown to calculate using mol = conc × vol.

Question 8

What is a concordant titre?

Answer 8

Two or more titres that are within 0.1 cm³ of each other.

Question 9

How many concordant titres should you use to find a mean value?

Answer 9

At least two concordant titres.

Question 10

Why do you rinse the burette and pipette with the solutions they will contain?

Answer 10

To avoid contamination and dilution of the solutions.

Question 11

What safety precautions should you take during titrations?

Answer 11

Wear safety goggles, handle acids carefully, and use a burette clamp securely.

Question 12

Describe the method for preparing a pure dry sample of a soluble salt.

Answer 12

React an acid with a base, filter to remove excess solid, evaporate solution to crystallize salt, then dry crystals.

Question 13

What is the purpose of filtration in salt preparation?

Answer 13

To remove excess insoluble reactant or impurities.

Question 14

How do you know when a salt solution is saturated?

Answer 14

When crystals start to form or the solution cannot dissolve any more solute.

Question 15

What is the technique for measuring the rate of reaction using gas collection?

Answer 15

Measure the volume of gas produced over time using a gas syringe or inverted measuring cylinder.

Question 16

How can you measure the rate of reaction by change in mass?

Answer 16

Use a balance to record the mass loss as gas escapes from the reaction vessel.

Question 17

Why is it important to measure time accurately in rate experiments?

Answer 17

Because rate depends on how quickly reactants/products change with time.

Question 18

What is the effect of temperature on reaction rate?

Answer 18

Increasing temperature increases reaction rate by providing more energy for successful collisions.

Question 19

What is the effect of concentration on reaction rate?

Answer 19

Increasing concentration increases the frequency of collisions, thus increasing rate.

Question 20

How do you carry out the iodine clock reaction practical?

Answer 20

Mix solutions of iodine and thiosulfate and time how long it takes for the blue-black starch-iodine complex to appear.

Question 21

What safety precautions are needed when handling iodine solutions?

Answer 21

Use gloves and avoid inhalation or ingestion; iodine is harmful.

Question 22

Describe the method to prepare a pure

Answer 22

dry sample of an insoluble salt.

Question 23

How do you confirm the identity of a gas evolved in a reaction?

Answer 23

Test with appropriate methods: pop test for hydrogen, limewater for carbon dioxide, glowing splint for oxygen.

Question 24

What is the importance of using a volumetric flask in preparing solutions?

Answer 24

It allows accurate preparation of solutions of known concentration.

Question 25

How do you calibrate a volumetric flask?

Answer 25

Fill to the mark with distilled water at a controlled temperature (usually 20°C).

Question 26

What is the typical procedure for crystallization in salt preparation?

Answer 26

Heat solution to evaporate some solvent, cool to allow crystals to form, then filter and dry crystals.

Question 27

What is the difference between accuracy and precision in measurements?

Answer 27

Accuracy is how close a measurement is to the true value; precision is how close repeated measurements are to each other.

Question 28

What is the role of a catalyst in a reaction?

Answer 28

To provide an alternative pathway with lower activation energy, increasing the rate without being consumed.

Question 29

How do you test for purity of a solid sample?

Answer 29

Measure melting point and compare to known values; pure substances have sharp melting points.

Question 30

Why is it important to dry crystals carefully?

Answer 30

To avoid loss of sample and to obtain an accurate mass.

Question 31

Describe how you would determine the molar volume of a gas at room temperature and pressure.

Answer 31

Measure volume of gas produced from reaction at RTP using gas syringe and calculate using mol = volume/24.

Question 32

How can you measure the concentration of a solution using colorimetry?

Answer 32

Measure absorbance of solution and compare against a calibration curve.

Question 33

What is a calibration curve?

Answer 33

A graph of absorbance against concentration for known standards.

Question 34

How do you prepare a calibration curve?

Answer 34

Prepare standard solutions of known concentrations, measure absorbance, and plot results.

Question 35

What common sources of error exist in titrations?

Answer 35

Parallax error reading burette, inaccurate endpoint detection, not rinsing equipment.

Question 36

What is the role of washing the precipitate in filtration?

Answer 36

To remove impurities or soluble reactants trapped in precipitate.

Question 37

How do you measure the rate of reaction by monitoring color change?

Answer 37

Use a colorimeter to measure change in absorbance over time.

Question 38

What is the reaction equation for the reaction between sodium thiosulfate and hydrochloric acid?

Answer 38

Na₂S₂O₃ + 2HCl → 2NaCl + SO₂ + S + H₂O

Question 39

How do you calculate the rate of reaction from a concentration vs time graph?

Answer 39

Calculate the gradient (change in concentration/change in time) at a particular point.

Question 40

How do you calculate the percentage uncertainty of a measurement?

Answer 40

(Absolute uncertainty ÷ measured value) × 100

Question 41

Why is it important to control variables during rate experiments?

Answer 41

To ensure that only the independent variable affects the reaction rate, ensuring valid results.

Question 42

What is the principle of the iodine clock reaction?

Answer 42

The time taken for iodine to react with thiosulfate until it is free to react with starch, producing a sudden color change.

Question 43

What common mistakes should be avoided in preparing a standard solution?

Answer 43

Not dissolving the solid fully, not making up to the mark accurately, contamination of solution.

Question 44

How do you ensure safety when handling acids and alkalis in practicals?

Answer 44

Wear goggles and gloves, add acid to water (not vice versa), work in a fume hood if needed.

Question 45

What is the significance of using a white tile in color change observations?

Answer 45

It provides a consistent background to clearly observe color changes.

Question 46

How do you prepare a pure dry sample of a hydrated salt?

Answer 46

Allow crystals to form by evaporation, then dry gently to preserve water of crystallization.

Question 47

What is the importance of repeating measurements in practicals?

Answer 47

To reduce random error and increase reliability of results.

Question 48

What is the end point in a titration?

Answer 48

The point at which the indicator changes colour, indicating stoichiometric neutralization.

Question 49

How can you improve the accuracy of a titration?

Answer 49

Use a suitable indicator, read burette at eye level, perform multiple concordant titres.

Question 50

What safety equipment is essential for all chemistry practicals?

Answer 50

Safety goggles, lab coat, gloves, and access to eye wash and safety shower.

Question 51

Describe tests to identify hydrogen, oxygen, and carbon dioxide.

Answer 51

Hydrogen: Insert a lit splint into the gas – it gives a squeaky pop. Oxygen: Insert a glowing splint – it relights. Carbon dioxide: Bubble the gas through limewater – it turns milky/cloudy due to formation of CaCO₃.

Question 52

What is the test for chlorine gas? and positive result?

Answer 52

Hold damp blue litmus paper near the gas – it turns red and then white (bleached). The red colour shows chlorine is acidic. The white/bleached paper confirms chlorine’s oxidising and bleaching action.

Question 53

Describe how to make up 250 cm³ of a standard solution of known concentration from a solid. (6 marks)

Answer 53

Weigh the required mass of solid using a balance and a weighing boat. Transfer the solid into a beaker and rinse the boat with distilled water into the beaker. Add distilled water to the beaker and stir with a glass rod until all the solid dissolves. Use a funnel to pour the solution into a 250 cm³ volumetric flask. Rinse the beaker, funnel, and stirring rod with distilled water and add the rinsings to the flask. Make up to the mark with distilled water, then stopper and invert the flask several times to mix.

Question 54

Describe how to carry out a titration to find the concentration of an acid. (6 marks)

Answer 54

Use a pipette and pipette filler to transfer 25.0 cm³ of alkali into a conical flask. Add a few drops of a suitable indicator (e.g., phenolphthalein or methyl orange). Fill a burette with standard acid solution, record the initial volume. Slowly add acid from the burette to the flask while swirling, until the indicator changes colour. Record the final volume and calculate the titre. Repeat until you get concordant results (within 0.10 cm³), then calculate the mean titre from these.

Question 55

Describe a method to measure the enthalpy change of neutralisation. (6 marks)

Answer 55

Use a polystyrene cup placed in a beaker for insulation, with a lid and a thermometer. Measure a known volume (e.g., 50 cm³) of acid into the cup and record the initial temperature. Measure an equal volume of alkali separately. Add the alkali to the acid quickly, stir with a thermometer, and record the temperature at regular intervals. Note the highest temperature reached after mixing. Calculate the temperature change and use q = mcΔT to determine enthalpy change per mole.

Question 56

Describe how to test for the presence of sulfate, halide, and carbonate ions. (6 marks)

Answer 56

To test for sulfate ions: add dilute HCl, then BaCl₂ solution – a white precipitate of BaSO₄ confirms sulfate. To test for halides: add dilute nitric acid, then AgNO₃ solution – observe precipitate colour: Cl⁻: white Br⁻: cream I⁻: yellow Test for carbonates: add dilute HCl – effervescence indicates CO₂, which turns limewater milky. Carry out these tests in sequence to avoid false positives. Always use a fresh sample or rinse thoroughly between tests. Use flame tests for metal cations if required, e.g., Na⁺ gives a yellow flame.

Question 57

Describe how to measure the rate of reaction between marble chips and HCl. (6 marks)

Answer 57

Place a known mass of marble chips in a conical flask. Add a known volume and concentration of hydrochloric acid. Quickly place a cotton wool plug or attach a gas syringe (depending on method chosen). Start a stopwatch and record the mass lost (or volume of gas collected) at regular time intervals. Continue until the reaction finishes (no more mass loss or gas evolved). Plot the data as a graph of mass/time or volume/time; rate = gradient of the tangent at a point

Question 58

Describe how to identify an unknown organic compound using functional group tests. (6 marks)

Answer 58

Test for alkenes: Add bromine water – decolourisation confirms a C=C bond. Test for alcohols: Add acidified potassium dichromate and warm – orange to green shows a primary or secondary alcohol. Test for aldehydes: Add Tollens’ reagent and warm – a silver mirror confirms an aldehyde. Test for carboxylic acids: Add Na₂CO₃ – effervescence confirms COOH group. Use IR spectroscopy to identify functional groups by characteristic absorptions. Compare melting/boiling points and solubility with known substances to help confirm identity.