T&P Analysis

Question 1

How is the R_f value in chromatography calculated?

Answer 1

d_spot / d_solvent

Question 2

Describe how paper chromatography is carried out.

Answer 2

• Draw pencil (insoluble) line 1 cm from base of chromatography paper (stationary phase)
• Using capillary tubes, spot test mixture + pure reference samples along line
• Suspend plate in beaker containing solvent reaching max height 5 mm
• Cover with watch glass (prevents evaporation)
• Remove paper when solvent front nears the top; mark point reached + allow to dry
• Measure height of spots + calculate R_f values = d_spot/d_solvent
• Match with those of known compounds in this solvent

Question 3

What is thin layer chromatography used to do?

Answer 3

• Separate small quantities of organic compounds
• Purify them / check their purity
• Follow the progress of a reaction

Question 4

Thin layer chromatography is used to:

• Separate small quantities of organic compounds
• Purify them / check their purity
• Follow the progress of a reaction

Describe the principle upon which this technique is based.

Answer 4

• Organic compounds have different affinities for a given solvent
• So are carried through the chromatography (silica / alumina) plate at different rates

Question 5

Describe how thin layer chromatography is carried out.

Answer 5

• Draw pencil (insoluble) line 1 cm from base of silica (SiO₂) / alumina (Al₂O₃) plate (stationary phase)
• Using capillary tubes, spot test mixture + pure reference samples along line
• Suspend plate in beaker containing solvent reaching max height 5 mm
• Cover with watch glass (prevents evaporation)
• Remove plate when solvent front nears the top; mark point reached + allow to dry
• Locate spots with UV lamp
• Measure height of spots + calculate R_f values = d_spot/d_solvent
• Match with those of known compounds in this solvent

Question 6

What can be used to locate spots in TLC?

Answer 6

• UV lamps
• Iodine
• Ninhydrin

Question 7

Describe how you would carry out a titration to find the concentration of a sodium hydroxide solution, using 0.2 mol dm^-3 ethanoic acid.

Describe how you would use the titre to calculate the concentration of the sodium hydroxide solution.

Answer 7

• Rinse + fill burette with CH₃COOH(aq)
• Record initial reading to nearest 0.05 cm³
• Rinse + fill graduated pipette with NaOH(aq)
• Use pipette filler to add 25 cm³ to a conical flask
• Add indicator to flask
• Perform rough titration
• Repeat, adding acid dropwise when 1 cm³ below end-point
• Repeat to obtain 3 concordant titres within 0.1 cm³
• Calculate average titre
• [NaOH] = mol / volume = (0.2 x titre x 10^-3) / (25 x 10^-3)

Question 8

A student performs a titration of sodium thiosulfate (in the burette) and potassium iodate(V) (in the conical flask) and gets a titre of 0.48 cm³.

1. Why is their teacher disappointed?
2. How could their method be improved?

Answer 8

1. Large % uncertainty due to small titre (21%)
2. Use more dilute thiosulfate or more concentrated iodate(V)

Question 9

What are iodine-thiosulfate titrations used for?

Answer 9

Finding the concentration of a strong enough oxidising agent to oxidise I^- to I_2.

Question 10

An iodine-thiosulfate titration can be used to find the concentration of a solution of chlorate(I) ions, which react as follows:

A: ClO^- + I^- + H⁺ →I₂ + Cl^- + H₂O

B: S₂O₃^2- + I₂ → I^- + S₄O₆^2-

1. Balance equations A and B.
2. Describe how the titration would be carried out, but do not include details of the procedure. Refer to equations A and B in your answer. Include details of the observations which would be made.

Answer 10

1.

A: ClO^- + 2I^- + 2H⁺ →I₂ + Cl^- + H₂O

B: 2S₂O₃^2- + I₂ → 2I^- + S₄O₆^2-

2.

• Add excess I^-(aq) to ClO^-(aq). Forms brown mixture since iodine is formed in A.
• Titrate I₂ produced with thiosulfate ions. When mixture turns from brown to pale yellow, add a few drops of starch solution. End-point is when blue-black colour, due to iodine-starch complex, disappears, since all I₂ has been reduced, as in B.

Question 11

An iodine-thiosulfate titration can be used to find the concentration of a solution of chlorate(I) ions, which react as follows:

A: ClO^- + 2I^- + 2H⁺ →I₂ + Cl^- + H₂O

B: I₂ + 2S₂O₃^2- → 2I^- + S₄O₆^2-​

Describe how a titration would be carried out to find the concentration of a solution of chlorate(I) ions, using standard 0.1 mol dm^-3 sodium thiosulfate solution. Include details of the observations which would be made.

Answer 11

A: ClO^- + 2I^- + 2H⁺ →I₂ + Cl^- + H₂O

• Rinse + fill graduated pipette with ClO^-(aq)
• Use pipette filler to transfer 25 cm³ to conical flask
• Add excess KI(aq)
• Add excess H₂SO₄(aq)
• Mixture turns brown since iodine forms

B: I₂ + 2S₂O₃^2- → 2I^- + S₄O₆^2-​

• Rinse + fill burette with standard 0.1 mol dm³ S₂O₃^2-(aq)
• Run some out to remove bubbles in jet
• Record initial reading to nearest 0.05 cm³
• Place conical flask onto white tile
• Perform rough titration
• When mixture turns pale yellow, add a few drops of starch solution
• End-point is when mixture turns from blue-black, due to iodine-starch complex, to colourless
• Repeat to get accurate titres, adding dropwise when 1 cm³ below rough end-point
• Repeat to obtain 3 concordant titres within 0.1 cm³
• Calculate average titre
• [ClO^-] = mol / vol = (0.1 x titre x 10^-3 x 1/2) / (25 x 10^-3)

Question 12

The concentration of a solution of Fe²⁺ ions can be found by titration with a standard potassium manganate(VII) solution. Other than from the uncertainty in burette and mass balance readings, suggest potential sources of error in the prodedure.

Answer 12

• Loss of material during transfer between vessels
• Parallax error in reading burette, volumetric flask + pipette
• Drips inside burette increasing titre
• Random error in judging end-point; titre may be too small or large

Question 13

A solution of Fe²⁺ ions is made up by dissolving a certain mass of thyme in water. Some of the solution formed is transferred to a conical flask.

A student wants to determine the iron content of thyme. They do this by finding the concentration of the Fe²⁺ solution, by titration with standard potassium manganate(VII) solution.

Describe how the percentage uncertainty of the method could be minimised.

Answer 13

• Use a greater mass of thyme, in order to:
  
  • reduce % uncertainty in mass balance reading
  • make Fe²⁺(aq) more concentrated, to achieve a larger titre, to reduce % uncertainty in burette reading
• Use more dilute manganate(VII) solution to achieve a larger titre, to reduce % uncertainty in burette reading

Question 14

The concentration of a solution of Fe²⁺ ions can be found by titration with a standard potassium manganate(VII) solution.

• Describe how the titration would be carried out.
• State how the end-point can be recognised and explain why an indicator does not need to be added.

Answer 14

• Rinse + fill burette with standard MnO₄^-(aq)
• Rinse + fill graduated pipette with Fe²⁺(aq)
• Use filler to transfer 25 cm³ to conical flask
• Add excess H₂SO₄ to flask
• Titrate until pink colour remains due to excess MnO₄^-(aq); no indicator required
• Repeat to obtain 3 concordant results within 0.1 cm³

Don’t use HCl as easily oxidised to toxic Cl₂

Question 15

Answer 15

1. 2 x 0.05 x 100 / 24.7 = 0.4%
2. Student A correct; titre unaffected since mol Sr(OH)₂ unaltered

Question 16

• What 2 instruments can be used to determine the concentration of a coloured solution?
• Distinguish between the two.

Answer 16

• Colorimeter (gives data for restricted wavelengths of light due to specific filters)
• Visible spectrophotometer (gives data for any visible wavelength)

Question 17

Describe how a colorimeter would be used to measure the concentration of a sample.

Answer 17

• Select filter of complementary colour to sample
• Zero colorimeter with tube of pure solvent (usually water)
• Prepare range of 5+ standard test solutions, with concentrations above and below that of sample
• Measure absorbance of standard solutions + sample of unknown concentration
• Plot calibration curve of concentration against absorbance for samples of known concentration
• Read off unknown concentration

Question 18

A student has test tubes containing solutions of 0.010 mol dm^-3, 0.020 mol dm^-3, 0.030 mol dm^-3, and 0.040 mol dm^-3 of MnO₄^- ions.

How could they use a colorimeter to measure the concentration of a solution containing approximately 0.025 mol dm^-3 MnO₄^- ions?

Answer 18

• MnO₄^-(aq) is purple so select yellow (complementary colour) filter
• Zero colorimeter with tube of pure water
• Use colorimeter to measure absorbance of all MnO₄^- solutions, including sample of unknown concentration
• Plot calibration curve of concentration against absorbance for samples of known concentration
• Read off unknown concentration

Question 19

The standard electrode potential of a half-cell may be measured by connecting it to which two types of half-cell?

Answer 19

• Standard hydrogen half-cell
• Calibrated reference half-cell (calibrated with H₂ half-cell)

Question 20

Draw a diagram of the apparatus used to determine the standard electrode potential of a Zn(s)/Zn²⁺(aq) half-cell.

Answer 20

Also: H₂ half-cell electrode should be labelled as Pt(s) or graphite

Question 21

Describe a technique which could be used to determine the standard potential of a Fe³⁺(aq)/Fe²⁺(aq) half-cell.

Answer 21

• Set up Fe³⁺/Fe²⁺ half-cell:
  
  • Use platinum / graphite electrode
  • Fe³⁺ and Fe²⁺ in solution
• Set up standard hydrogen half-cell:
  
  • Use platinum / graphite electrode
  • H⁺ in solution
  • Feed H₂(g) at 100 kPa into solution using porous glass tube covering electrode
• Concentrations of solutions all 1 mol dm^-3
• Temperature of solutions and H₂(g) 298 K
• Connect solutions with salt bridge
• Connect electrodes with high-resistance voltmeter
• Record voltmeter reading (switch connections if negative)
• This gives E_cell and therefore E^o (since E^o for H₂ half-cell = 0 V)

Question 22

Answer 22

Advantage: portable / no gas involved

Disadvantage: mercury is toxic / poisonous

Ignore references to cost. Mercury being “harmful” is too vague

Question 23

Before using a pH probe (pH electrode), it must be calibrated for which two quantities?

Answer 23

• Temperature
• pH

Question 24

Describe how to calibrate a pH probe.

Answer 24

• Rinse probe with distilled water
• Immerse in buffer solution of pH 7.00
• Connect to meter
• Calibrate for temperature (if not automatic)
• Set meter pH reading to 7.00
• Re-rinse probe
• If intending to measure a wide range of pH values, calibrate with acidic + alkaline buffer solutions too

Question 25

Describe the test for unsaturation. State what would be observed as a positive result.

Answer 25

• Shake sample with bromine water
• Positive result: brown bromine water decolourises

Question 26

Draw a diagram of the apparatus used to crack a hydrocarbon vapour over a heated catalyst.

Answer 26

Also label delivery tube

Question 27

Describe how you would crack an alkane over a heated catalyst.

Answer 27

• Soak mineral wool with alkane + place at end of horizontally mounted boiling tube
• Add catalyst (porcelain chips) to middle of tube, leaving space for passage of gas
• Place bung in tube
• Fill collection trough with water
• Add several inverted test tubes
• Connect to boiling tube with delivery tube
• Use Bunsen flame to heat catalyst
• Collect gas in tubes, replacing + corking when full. Discard first tube; contains displaced air. Heat throughout - this prevents suck-back
• Remove delivery tube from trough before stopping heating - this prevents suck-back

Question 28

A student cracked liquid paraffin over a heated catalyst and collected the gaseous product in several test tubes.

Explain how they could test whether they had produced an alkene from an alkane.

Answer 28

• Shake liquid paraffin with bromine water: should remain brown
• Shake gas in tubes with bromine water: brown solution should decolourise

Question 29

Name 5 methods of analysis which are used to measure rate of reaction.

Answer 29

• Titration (redox / acid-base)
• Colorimetry
• pH measurement
• Volume of gas evolved
• Mass change

Question 30

How does quenching work?

Answer 30

• Sample isolated from reaction mixture
• Chemical added which stops reaction (e.g. neutralises a catalyst)
• Sample titrated to find conc. of a reactant/product

Question 31

Name 3 methods of analysis which are used to determine the value of an equilibrium constant.

Answer 31

• Titration (redox / acid-base)
• Colorimetry
• pH measurement

Question 32

Explain how the value of K_c can be found experimentally.

Answer 32

• Record initial reactant concentrations
• Allow mixture to reach equilibrium
• Measure eq conc. of a reactant or product
• Calculate eq conc. of other reactants + products using above + equation
• Write expression for K_c then calculate value using eq concentrations. State with unit + temperature

Question 33

The reaction below is done at 298 K. The initial reactant concentrations are known.

1. How would you find the concentration of ethanoic acid in the eq. mixture?
2. How would you find an expression for K_c?

Answer 33

1. Titrate a known volume of equilibrium mixture with standard NaOH solution

2.

• Assume [CH₃COOH]_eq = [CH₃CH₂OH]_eq
• Let this = x
• [CH₃COOCH₂CH₃]_eq and [H₂O]_eq must equal initial conc - x
• Let initial concentrations respectively be y and z
• K_c = x² / (y - x)(z - x) at 298 K (no unit)

Question 34

Describe how you would determine the solubility product, K_sp, of a soluble salt.

Answer 34

Prepare a saturated solution:

• Swirl salt into conical flask containing warm distilled water until no more dissolves
• Cool
• Check there is undissolved solid at bottom of flask; if not, repeat first step
• Filter

Measure temperature (K_sp is temperature-dependent)

Find concentration of one of the dissolved ions (method depends: e.g. titration, colorimetry)

Use equation to determine concentration of other ion

Calculate K_sp, giving unit + temperature

Question 35

Describe how you could determine the value of K_sp for calcium hydroxide at 298 K.

Answer 35

Prepare a saturated solution:

• Swirl Ca(OH)₂ into conical flask containing warm distilled water until no more dissolves
• Cool
• Check there is undissolved solid at bottom of flask; if not, repeat first step
• Filter

Titrate known volume of filtrate with hydrochloric acid. Calculate [OH^-]

[Ca²⁺] = [OH^-] / 2

K_sp = [Ca²⁺][OH^-]², in mol³ dm^-12 at 298 K