Practical skills Flashcards

Question

Describe a method of using a gas syringe to calculate the Mr of propanone

Answer 1

Extract 0.20cm^3 of propanone into a hypodermic syringe and then measure the mass of the syringe Using hand protection, remove a gas syringe from the oven and note the volume of air already in the barrel inject the propanone though the self-seal cap into the barrel. the plunger will move straight away. Put the gas syringe back into the oven Measure the mass of the empty hypodermic syringe immediately 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

Answer 2

Pipette 25cm^3 of the original solution into a 250cm^3 volumetric flask Makeup to the mark with distilled water using a pipette for the last few drops Invert the flask several times to ensure a uniform solution

Answer 3

pink in alkali | colourless in acid

Answer 4

red in acid | yellow in alkali

Answer 5

The acid or alkali added may be diluted by residual water or may react with substances left from a previous titration. The concentration of the substance would be lowered and a larger titre would be used.

Answer 6

the water does not react with the reagents or change the number of moles of the acid/alkali added

Answer 7

MnO4- + 8H+ + 5Fe2+ → Mn2+ + 4H2O + 5Fe3+

Answer 8

the titration is self indicating because of the significant colour change from reactant to product

Answer 9

Weigh accurately two ferrous sulphate tablets Grind up the tablets with a little 1M sulphuric acid, using a pestle and mortar Through a funnel, transfer the resulting pastel into a 100cm^3 volumetric flask. Use further small volumes of 1M of sulphuric acid to rinse the ground up tablets into the flask Then add sufficient 1M sulphuric acid to make up the solution to exactly 100cm^3. Stopper the flask and shake it to make sure that all the contents are thoroughly mixed Titrate 10cm^3 portions of the solution with 0.0050M potassium manganate

Answer 10

the solution would not be acidic enough MnO2 will be produced MnO4- 4H+ + 3e- → MnO2 + 2H2O the brown MnO2 will mask the colour change and lead to a greater volume of manganate being used in the titration

Answer 11

the Cl- ions will be oxidised to Cl2 by the manganate ions | a greater volume of manganate will be used and poisonous Cl2 will be produced

Answer 12

It is an oxidising agent. The Fe2+ ions will be oxidised to Fe3+ ions a smaller volume of manganate will be used

Answer 13

2MnO4(-) + 6H+ + 5H2O2 → 5O2 + 2Mn(2+) + 8H2O

Answer 14

2MnO4(-) + 16H+ + 5C2O4(2-) + 10CO2 + 2Mn(2+) + 8H2O

Answer 15

the reaction is between two negative ions

Answer 16

there is a discrepancy in the result due to other errors

Answer 17

there is no discrepancy and all errors in the results can be explained by the sensitivity of the equipment

Answer 18

Add a known volume of acid to an insulated container and measure the temperature Then add a known volume of alkali and record the temperature of the mixture at regular intervals over a period of time Find the temperature change for the experiment. Use it to calculate the enthalpy change of the reaction.

Answer 19

Record the temperature at regular intervals before the start of the reaction Plot the temperature against the time Extrapolate the graph to when the reactants are added together

Answer 20

- energy transfer from surroundings - the method assumes that all solutions have the same specific heat capacity of water - the specific heat capacity of the calorimetry is neglected - incomplete reaction - density of the solution is taken to be the same as water

Answer 21

-Weigh out between 3.90g and 4.10g of anhydrous copper(II) sulphate in a dry weighing bottle. Use weighing by difference to record an accurate mass -Using a volumetric pipette, place 25cm^3 of deionised water into a polystyrene cup and record its temperature at the beginning, start the timer and then record the temperature again every minute,stirring the liquid continuously. -At the forth minute, add the powdered anhydrous copper(II) sulphate rapidly to the polystyrene cup and stir. Record the temperature every minute up to 15 minutes continuously stirring the mixture Plot a graph of temperature against time Use the graph to find the temperature change use q=mc(change in)T chang in H = q/moles

Answer 22

It is impossible to add the exact amount of water without the solid dissolving and it is not easy to measure the temperature change of a solid

Answer 23

- energy losses from calorimetry - incomplete combustion of the fuel - incomplete transfer of energy - evaporation of fuel after weighing - heat capacity of calorimeter not accounted for - measurements not carried out under standard conditions

Answer 24

Measure out fixed volumes of sodium thiosulphate hydrochloric acid using a measuring cylinder Use a water bath to gently heat both solutions to the desired temperature before you mix them Mix the solutions in a conical flask. Place the flask over a black cross which can be seen through the solution. Watch the black cross disappear through the cloudy sulfur and time how long it takes to disappear. Repeat the experiment for different temperatures keeping the volume and concentrations the same

Answer 25

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

Answer 26

Add 10 drops of 0.1 moldm^3 of the metal ion solution in a test tube. Add about 10 drops of 0.6moldm^3 of sodium hydroxide into the test tube. Add sodium hydroxide until excess OR Add 10 drops of 0.1 moldm^3 of the metal ion solution in a test tube. Add about 10 drops of 1.0moldm^3 of sulphuric acid into the test tube. Add sulphuric acid until excess

Answer 27

the solubility of the hydroxides increases down group 2 | magnesium and calcium solutions would produce white precipitates

Answer 28

the solubility of the sulphates decreases down the group | strontium and barium solutions produce white precipitates

Answer 29

- Add 10 drops of 0.1 moldm^3 of ammonium chloride in a test tube. - Add about 10 drops of 0.4 moldm^3 of sodium hydroxide solution into the test tube and shake the mixture. - Warm the mixture in the test tube gently using a water bath - Test the fumes released from the mixture by holding a piece of damp red litmus paper in the mouth of the test tube. - Alkaline ammonium gas turns the red litmus paper blue

Answer 30

BaCl2 solution acidified with HCl | white precipitate of BaSO4 formed

Answer 31

The HCl reacts with carbonate impurities that may give a false positive result

Answer 32

acidified silver nitrate solution added dropwise into a container containing halide ions

Answer 33

Flourides - no precipitate Chlorides - white precipitate Bromides - cream precipitates Iodides - yellow precipitates

Answer 34

Silver chloride dissolves in dilute ammonia Silver bromide dissolves in concentrated ammonia Silver iodide is insoluble

Answer 35

Add any dilute acid and observe effervescence | Bubble the gas through limewater to test for carbon dioxide- the limewater will turn cloudy

Answer 36

Alkaline hydroxide ions will turn red litmus paper blue

Answer 37

NaF + H2SO → NaHSO4(s) + HF(g) Observations: White steamy fumes of HF NaCl(s) + H2SO4(l) → NaHSO4(s) + HCl(g) Observations: White steamy fumes of HCl

Answer 38

``` NaBr + H2SO4 → NaHSO4 + HBr 2HBr + H2SO4 → Br2+ SO2+ 2H2O Observations: white steamy fumes of HBr red fumes of Bromine colourless, acidic gas of sulphur dioxide ```

Answer 39

``` NaI + H2SO4→ NaHSO4 + HI 2HI + H2SO4 → I2 + SO2 + 2H2O 6HI + H2SO4 → 3 I2 + S + 4 H2O 8HI + H2SO4 → 4I2 + H2S + 4H2O Observations: white steamy fumes of HI black sold and purple fumes of iodine colourless, acidic gas of SO2 yellow solid of sulphur H2S, a gas with a rotten egg smell ```

Answer 40

acidified potassium dichromate orange to green

Answer 41

-Place about 10 cm3 of dilute sulfuric acid in a flask and add about 3g of potassium dichromate(VI) and 2 or 3 anti- bumping granules. Shake the contents of the flask until solution is complete. -Add 1.5 cm3 of propan-1-ol in drops from a dropping pipette, shaking the flask so as to mix the contents -Use a distillation apparatus to carry out the experiment -Gently heat and slowly distil 2 cm3 of liquid into a test tube, taking care that none of the reaction mixture splashes over

Answer 42

organic chemicals are normally highly flammable and could be set on fire with a naked flame

Answer 43

acidified potassium dichromate orange to green heat under reflux

Answer 44

-Measure 5 cm3 of water into a boiling tube. Add 6 g of sodium dichromate(VI), shake and set aside to dissolve. -Put about 1.5 cm3 propan-1-ol into a 50 cm3 round bottomed flask and add about 5 cm3 of water and two or three anti-bumping granules. Put a condenser on the flask for reflux. -Add 2 cm3 of concentrated sulfuric acid down the condenser in drops from a dropping pipette. While the mixture is still warm, start to add your sodium dichromate(VI) solution down the condenser in drops from a dropping pipette. The energy released from the reaction should make the mixture boil. -Add the solution a drop at a time so that the mixture continues to boil without any external heating. -When all the sodium dichromate(VI) solution has been added, use a low Bunsen burner flame to keep the mixture boiling for 10 minutes, not allowing any vapour to escape. -At the end of that time remove the Bunsen burner and arrange the apparatus for distillation. Gently distil 2-3 cm3 of liquid into a test tube.

Answer 45

as gas builds up, the pressure could cause the apparatus to explode

Answer 46

To prevent vigorous, uneven boiling by making smaller bubbles

Answer 47

Heat the flask, with a Bunsen burner causing the vapours of all the components in the mixture to be produced. Vapours pass up the fractionating column. The vapour of the substance with the lower boiling point reaches the top of the fractionating column first. The thermometer should be at or below the boiling point of the most volatile substance. • The vapours with higher boiling points condense back into the flask. Only the most volatile vapour passes into the condenser. The condenser cools the vapours and condenses to a liquid and is collected

Answer 48

Bromine water | observation: orange to colourless

Answer 49

Aldehyde Tollen's reagent - silver mirror Fehling's solution - brick red precipitate Ketones no visible change for both tests

Answer 50

Add sodium carbonate to a solution | Effervescence would be observed if a carboxylic acid was present as carbon dioxide is produced

Answer 51

It is the rate right at the start of the reaction. This can be found by calculating the gradient of the tangent at t=0.

Answer 52

Repeat an experiment several times using different initial concentrations of the reactions, changing of the concentrations at a time, keeping the rest constant. Calculate the initial rate of each experiment Use the initial rates and concentrations to work out the orders of each reactant to work out the rate equation.

Answer 53

H2O2(aq) + 2H+(aq) + 2I–(aq) → I2(aq) + 2H2O(l) | 2S2O3(2-)(aq) + I2(aq) → 2I(–)(aq) + S4O6(2–)(aq)

Answer 54

measure: - the change in volume of a gas - the change of mass - colorimetry - the change in electrical conductivity - optical activity

Answer 55

-Clean the zinc and copper foils with emery before use. -Degrease the metal using some cotton wool and propanone. -Place the copper strip into a 100 cm3 beaker with about 50 cm3 of 1 mol dm–3 CuSO4 solution. -Place the zinc strip into a 100 cm3 beaker with about 50 cm3 of 1 mol dm–3 ZnSO4 solution. -Use a strip of filter paper soaked in saturated potassium nitrate solution for the salt bridge -Connect the Cu(s)|Cu2+ (aq) and Zn(s)|Zn2+ (aq) half- cells by connecting the metals using the crocodile clips and leads provided to the voltmeter

Answer 56

A piece of filter paper soaked in potassium nitrate

Answer 57

allows for the transfer of ions to balance the charge

Answer 58

1. Transfer 25cm3 of acid to a conical flask with a volumetric pipette 2. Measure initial pH of the acid with a pH meter 3. Add alkali in small amounts (2cm3 ) noting the volume added 4. Stir mixture to equalise the pH 5. Measure and record the pH to 1 d.p. 6. Repeat steps 3-5 but when approaching endpoint add in smaller volumes of alkali 7. Add until alkali in excess

Answer 59

-Dissolve the impure solid in a minimum volume of a hot solvent- this should give a saturated solution of the impure solid Filter the hot solution through a heated funnel to remove any insoluble impurities Leave the solution to cool down slowly. Crystals of the product will form as as it cools Remove the liquid containing the soluble impurities from the crystals by filtering the mixture under reduced pressure. Pour the mixture into a filter paper lined Buchner funnel, that's sitting in a side arm flask attached to a vacuum line. Finally, wash the crystals with ice-cold solvent to remove any soluble impurities from their surface. Leave the purified crystals to dry.

Answer 60

Pure substances have specific melting and boiling points. Impurities lowers the melting point and the boiling point is raised. The melting and boiling points occur across a wide range of temperatures.

Answer 61

- Pack a small sample of the solid into a glass capillary tube and place it inside the heating element of the melting point apparatus. - Increase the temperature until the sample turns from solid to liquid - Measure the melting range from where the solid begins to melt to where it completely melts. - Compare the measured melting point to the data values - Impurities lower the melting point and broaden the melting range

Answer 62

Add to a 50 cm3 pear-shaped flask 2.0 g of 2-hydroxybenzoic acid and 4 cm3 of ethanoic anhydride. To this mixture add 5 drops of 85% phosphoric(v) acid and swirl to mix. Fit the flask with a reflux condenser and heat the mixture on a boiling water bath for about 5 minutes. Without cooling the mixture, carefully add 2cm3 of water in one portion down the condenser. When the vigorous reaction has ended, pour the mixture into 40 cm3 of cold water in a 100 cm3 beaker, stir and rub the sides of the beaker with a stirring rod necessary to induce crystallisation and, finally, allow the mixture to stand in an ice bath to complete crystallisation. Collect the product by suction filtration and wash it with a little water.

Answer 63

Using a measuring cylinder, measure out 15 cm3 of ethanol into a boiling tube. Prepare a beaker half-filled with hot water from a kettle at a temperature of approximately 75 °C. Use a spatula to add the crude aspirin to the boiling tube with ethanol and place the tube in the beaker of hot water. Stir the contents of the boiling tube until all of the aspirin dissolves into the ethanol. Pour the hot solution containing dissolved aspirin through a warmed filter funnel and fluted filter paper to hot filter Then pour filtrate into 40 cm3 of water in a conical flask. Allow the conical flask to cool slowly and white needles of aspirin should separate. Cool the whole mixture in an ice bath. Filter off the purified solid under reduced pressure and allow it to dry on filter paper. Record the mass of the dry purified solid

Answer 64

Pour the mixture into a separating funnel and add water Shake the funnel and then allow it to settle The organic layer and aqueous layer are immiscible and so separate out into two distinct layers then open the tap and run each layer off into a separate container

Answer 65

Take an organic solvent in which the product is more soluble than it is in water. Add it to the impure product solution and shake well The product will dissolve into the organic solvent, leaving the impurities dissolved in the water. The solvent containing the product can then be run off using a separating funnel

Answer 66

Add an anhydrous salt such as magnesium sulphate. The salt is used as a drying agent-it binds to any water present to become hydrated. first, add the salt to the organic layer and it will clamp together. Keep adding the drying agent until disperses evenly when you swirl the flask. Filter the mixture to remove the solid drying agent- place a piece of filter paper into a funnel that feeds into a flask and pour the mixture into the filter paper

Answer 67

a) Pour about 20 cm3 of cyclohexanol into a weighed 50 cm3 pear-shaped flask. Reweigh the flask and record the mass of cyclohexanol. b) Using a plastic graduated dropping pipette, carefully and with frequent shaking, add to the flask approximately 8 cm3 of concentrated phosphoric acid. c ) Add a few anti-bumping granules to the flask and assemble the distillation apparatus, so that the contents of the flask may be distilled. Heat the flask gently, distilling over any liquid which boils below 100 °C. d) Pour the distillate into a separating funnel and add 50 cm3 of saturated sodium chloride solution. Shake the mixture and allow the two layers to separate. e) run off the lower layer into a beaker and then transfer the upper layer, which contains the crude cyclohexene, into a small conical flask. f) Add a few lumps of anhydrous calcium chloride or anhydrous sodium sulfate(VI) or anhydrous magnesium sulfate to the crude cyclohexene to remove water. Stopper the flask, shake the contents and allow this to stand until the liquid becomes clear. g) Decant the liquid into a clean, dry, weighed sample container. h) Reweigh the container, calculate the mass of dry cyclohexene produced

Answer 68

1. Propan-1-ol (50 cm3) and ethanoic acid (50 cm3) are mixed thoroughly in a 250 cm3 round-bottomed flask. 2. Concentrated sulfuric acid (10 cm3) is added drop by drop to the mixture, keeping the contents of the flask well-shaken and cooled in an ice-water bath. 3. When the acid has all been added, a reflux condenser is fitted to the flask and the mixture gently boiled over an electric heating mantle for about 30 minutes. 4. The mixture is cooled, and the apparatus rearranged for distillation. The crude ester (about 60 cm3) is distilled off. 5. The distillate is placed in a separating funnel and shaken with about half its volume of 30% sodium carbonate solution, with the pressure being released at intervals. The lower aqueous layer is then discarded. 6. The crude ester is shaken in a separating funnel with about half its volume of 50% calcium chloride solution, which removes unreacted alcohol. The lower layer is discarded. 7. The ester is run into a clean, dry flask containing some anhydrous calcium chloride and swirled. 8. The ester is filtered into a clean, dry flask, with a few anti-bumping granules, and distilled. The fraction boiling between 100°C and 103°C is collected.

Answer 69

a) Wearing gloves, draw a pencil line 1 cm above the bottom of a TLC plate and mark spots for each sample, equally spaced along line. b) Use a capillary tube to add a tiny drop of each solution to a different spot and allow the plate to air dry. c) Add solvent to a chamber or large beaker with a lid so that is no more than 1cm in depth d) Place the TLC plate into the chamber, making sure that the level of the solvent is below the pencil line. Replace the lid to get a tight seal. e) When the level of the solvent reaches about 1 cm from the top of the plate, remove the plate and mark the solvent level with a pencil. Allow the plate to dry in the fume cupboard. f) Place the plate under a UV lamp in order to see the spots. Draw around them lightly in pencil. g) Calculate the Rf values of the observed spots.

Answer 70

Rf value= distance moved by amino acid/ distance moved by solvent front

Answer 71

to prevent contamination from the hands to the plates

Answer 72

it will not dissolve in the solvent

Answer 73

too big a drop will cause different spots to merge

Answer 74

to prevent the evaporation of the toxic solvent

Answer 75

if the solvent is too deep, it will dissolve the sample spots from the plate

Answer 76

work in a fume cupboard if the solvent is toxic or flammable fumes from escaping the room

Answer 77

UV light | iodine- spots appear purple

Answer 78

Separation by chromatography depends on the balance between the solutility in the mobile phase and the retention in the stationary phase

Answer 79

Add an appropriate ligand to intensify the colour Make up solutions of known concentrations Measure absorption or transmission Plot graph of absorption against concentration Measure the absorption of an unknown solution and determine its concentration from the calibration curve

Answer 80

Measure out samples of the unknown metal ion solution into three separate test tubes in the first test tube, add sodium hydroxide solution dropwise using a pipette and record the observations. Then add until in excess and record the observations in the second test tube, add ammonia solution dropwise and record any changes. Then add the ammonia until in excess and record any changes In the third test tube, add sodium carbonate solution dropwise and record the observations

Answer 81

OH- or NH3 - blue precipitate Cu(OH)2(H2O)4 excess OH- - no change excess NH3 - deep blue solution [Cu(NH3)4(OH)2]2+ Na2CO3 - green-blue precipitate CuCO3

Answer 82

OH- or NH3 - green precipitate Fe(OH)2(H2O)4 excess OH- - no change excess NH3 - no change Na2CO3 - green precipitate FeCO3

Answer 83

OH- or NH3 - white precipitate Al(OH)3(H2O)3 excess OH- -colourless solution [Al(OH)4(H2O)2]- excess NH3 - no change Na2CO3 - white precipitate Al(OH)3(H2O)3

Answer 84

OH- or NH3 - brown precipitate Fe(OH)3(H2O)3 excess OH- - no change excess NH3 - no change Na2CO3 - brown precipitate Fe(OH)3(H2O)3