Chemistry 9701/31 — October/November 2019

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

questions

marks

120

minutes

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

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

In this experiment you will determine the concentration of a sample of hydrochloric acid. You will do this by measuring the volume of hydrogen produced when an excess of magnesium reacts with the acid.

\text{Mg(s)} + 2\text{HCl(aq)} \rightarrow \text{MgCl}_2\text{(aq)} + \text{H}_2\text{(g)}

FA 1 is magnesium powder, Mg.
FA 2 is hydrochloric acid, HCl.

(a)

Method

Weigh the container with FA 1. Record the mass.
Fill the tub with water to a depth of approximately $5\text{ cm}$ .
Fill the $250\text{ cm}^3$ measuring cylinder completely with water. Hold a piece of paper towel firmly over the top, invert the measuring cylinder and place it in the water in the tub.
Remove the paper towel and clamp the inverted measuring cylinder so that the open end is just above the base of the tub.
Use the $25\text{ cm}^3$ measuring cylinder to place $25.0\text{ cm}^3$ of FA 2 into the reaction flask, labelled X.
Check that the bung fits tightly in the neck of flask X, clamp flask X, and place the end of the delivery tube into the inverted $250\text{ cm}^3$ measuring cylinder.
Remove the bung from the neck of flask X. Tip all of FA 1 into flask X and replace the bung immediately. Remove the flask from the clamp and swirl to mix the contents.
Swirl the flask occasionally until no more gas is evolved. Replace the flask in the clamp.
Measure and record the final volume of gas in the measuring cylinder.
Weigh and record the mass of the container with any residual solid.
Calculate and record the mass of FA 1 used.

Keep FA 2 for use in Question 2.

(b)

Calculations

(i)

Calculate the number of moles of hydrogen gas produced.
(Assume $1\text{ mol}$ of gas occupies $24.0\text{ dm}^3$ at this temperature.)

\text{moles of H}_2\text{(g)} = \dots\dots\dots\dots\dots\dots\dots\dots\dots\dots\dots\dots\dots\text{ mol}

(ii)

Calculate the concentration of hydrochloric acid in FA 2.

\text{concentration of HCl in FA 2} = \dots\dots\dots\dots\dots\dots\dots\dots\dots\dots\dots\dots\dots\text{ mol dm}^{-3}

(iii)

In this experiment the magnesium powder was in excess.

Calculate the mass of magnesium powder needed for complete reaction with all the hydrochloric acid in $25.0\text{ cm}^3$ of FA 2.

\text{mass of Mg} = \dots\dots\dots\dots\dots\dots\dots\dots\dots\dots\dots\dots\dots\text{ g}

(c)

A student suggested two modifications to the method in (a) to give a more accurate value for the concentration.

For each suggestion, state whether you agree with the student and explain your answer.

Suggestion 1: Use magnesium ribbon rather than powdered magnesium; keep the rest of the experiment the same.

Suggestion 2: Use twice the mass of magnesium powder; keep the rest of the experiment the same.

(d)

Another student carried out the experiment in (a) but used less magnesium than that calculated in (b)(iii).

State and explain the effect this would have on the calculated concentration of hydrochloric acid in FA 2.

The rest of this paper

2 more questions

Q2Manipulation, Measurement and Observation · Presentation of Data and Observations · Analysis, Conclusions and Evaluation16M
Q3Qualitative Analysis · Analysis, Conclusions and Evaluation16M

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