Chemistry 9701/34 — October/November 2019

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

questions

marks

120

minutes

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

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

Quantitative Analysis

Read through the whole method before starting any practical work. Where appropriate, prepare a table for your results in the space provided.

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

Hydrogen peroxide decomposes in a reaction catalysed by manganese(IV) oxide.

2\text{H}_2\text{O}_2(\text{aq}) \rightarrow 2\text{H}_2\text{O}(\text{l}) + \text{O}_2(\text{g})

You will investigate this decomposition by measuring the volume of oxygen collected over a period of time. You will also use the volume of oxygen collected to calculate the concentration of the aqueous hydrogen peroxide.

FB 1 is aqueous hydrogen peroxide, $\text{H}_2\text{O}_2$ .
FB 2 is manganese(IV) oxide, $\text{MnO}_2$ .

(a)

Method

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 in the water just above the base of the tub.
Use the $50\text{ cm}^3$ measuring cylinder to place $30\text{ cm}^3$ of FB 1 and $20\text{ cm}^3$ of distilled water into the reaction flask labelled X.
Check that the bung fits tightly into the neck of flask X, clamp flask X and place the end of the delivery tube into the $250\text{ cm}^3$ measuring cylinder.
Remove the bung from the neck of flask X. Add all of the FB 2 into the hydrogen peroxide in the flask and replace the bung immediately. Start the stop-clock and leave it running until the end of the experiment.
Remove the flask from the clamp and swirl to mix the contents, then replace the flask in the clamp.
After 1 minute measure the volume of gas collected.
After 4 minutes from the start of the experiment measure the volume of gas collected.

Keep FB 1 for use in Question 3.

Results

(b)

Calculation

(i)

Use the volume of gas that you collected at 4 minutes to calculate the number of moles of hydrogen peroxide which had decomposed at this time.
(Assume $1\text{ mol}$ of gas occupies $24.0\text{ dm}^3$ at this temperature.)

\text{moles of H}_2\text{O}_2 = \text{.............................. mol}

(ii)

Assume all the $\text{H}_2\text{O}_2$ had decomposed in 4 minutes.

Calculate the initial concentration of $\text{H}_2\text{O}_2$ , in $\text{mol dm}^{-3}$ , in FB 1.

\text{initial concentration of H}_2\text{O}_2 = \text{.............................. mol dm}^{-3}

(c)

A student missed taking a reading at 1 minute so took a reading at 2 minutes instead. This student stated that after 2 minutes:

\text{rate of reaction} = \left( \frac{\text{volume of gas collected}}{2} \right)\text{ cm}^3\text{ minute}^{-1}

Is the student correct? Explain your answer.

(d)

Another student carried out the experiment in (a) but used twice the mass of manganese(IV) oxide.

State and explain what effect this would have on the results obtained.

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2 more questions

Q2Manipulation, Measurement and Observation · Presentation of Data and Observations · Analysis, Conclusions and Evaluation16M
Q3Qualitative Analysis · Manipulation, Measurement and Observation · Analysis, Conclusions and Evaluation · Presentation of Data and Observations15M

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