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9701/35

Chemistry 9701/35 — October/November 2013

Cambridge AS Level · Advanced Practical Skills · worked solutions for every part, with the mark scheme

3
questions
40
marks
120
minutes

Topics Presentation of Data and Observations · Analysis, Conclusions and Evaluation · Manipulation, Measurement and Observation · Qualitative Analysis

Q1Manipulation, Measurement and ObservationPresentation of Data and ObservationsAnalysis, Conclusions and EvaluationFree sample

You are provided with a solution of an organic acid which is known to be one of the following.

methanoic acid, HCOOH\text{HCOOH}
propanoic acid, C2H5COOH\text{C}_2\text{H}_5\text{COOH}
pent-2-enoic acid, CH3CH2CH=CHCOOH\text{CH}_3\text{CH}_2\text{CH}=\text{CHCOOH}

The solution was made by dissolving 1.85 g1.85\text{ g} of acid in 250 cm3250\text{ cm}^3 of solution.

You are to suggest the identity of the acid by finding its relative molecular mass, MrM_r, using a titration method.

FA 1 is the solution of the unknown organic acid.
FA 2 is 0.100 mol dm−30.100\text{ mol dm}^{-3} sodium hydroxide, NaOH\text{NaOH}.
phenolphthalein indicator

(a)

Method

  • Fill the burette with FA 2.
  • Pipette 25.0 cm325.0\text{ cm}^3 of FA 1 into a conical flask.
  • Titrate FA 1 with FA 2 using phenolphthalein as indicator.
  • Perform a rough titration and record your burette readings in the space below.

The rough titre is ............................ cm3\text{cm}^3.

  • Carry out as many accurate titrations as you think necessary to obtain consistent results.
  • Make certain any recorded results show the precision of your practical work.
  • Record, in a suitable form below, all of your burette readings and the volume of FA 2 added in each accurate titration.
7M
(b)

From your accurate titration results, obtain a suitable value to be used in your calculations. Show clearly how you obtained this value.

25.0 cm325.0\text{ cm}^3 of FA 1 required ................ cm3\text{cm}^3 of FA 2

1M
(c)

Calculations

Show your working and appropriate significant figures in the final answer to each step of your calculations.

5M
(i)

Calculate the number of moles of sodium hydroxide in the volume of FA 2 you have calculated in (b).

moles of NaOH\text{NaOH} = ................. mol

(ii)

One mole of any of the organic acids reacts with one mole of sodium hydroxide. Calculate the concentration, in mol dm−3\text{mol dm}^{-3}, of the acid in FA 1.

concentration of the acid in FA 1 = ................. mol dm−3\text{mol dm}^{-3}

(iii)

Calculate the concentration, in g dm−3\text{g dm}^{-3}, of the acid used to make solution FA 1.

concentration of the acid in FA 1 = ................. g dm−3\text{g dm}^{-3}

(iv)

Using your answers to (ii) and (iii), calculate the relative molecular mass, MrM_r, of the acid in FA 1.

MrM_r of the acid = .................

(v)

Suggest which of the acids, methanoic, propanoic or pent-2-enoic acid, is present in FA 1.

Acid present is ................................................

(vi)

Suggest a test that could be carried out to distinguish pent-2-enoic acid from methanoic acid and propanoic acid. Give the expected result of your test.

test .....................................................................................

expected result .....................................................................................

The rest of this paper

2 more questions
  • Q2Manipulation, Measurement and Observation · Presentation of Data and Observations · Analysis, Conclusions and Evaluation12M
  • Q3Qualitative Analysis · Analysis, Conclusions and Evaluation · Presentation of Data and Observations15M
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