Chemistry 9701/21 — October/November 2012

Cambridge AS Level · AS Level Structured Questions · worked solutions for every part, with the mark scheme

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Topics Atoms, Molecules and Stoichiometry · Equilibria · Hydroxy Compounds · Carboxylic Acids and Derivatives · Group 17 · Nitrogen and Sulfur · +5 more

Q1Atoms, Molecules and StoichiometryFree sample

Zinc is an essential trace element which is necessary for the healthy growth of animals and plants. Zinc deficiency in humans can be easily treated by using zinc salts as dietary supplements.

(a)

One salt which is used as a dietary supplement is a hydrated zinc sulfate, $\text{ZnSO}_4\cdot x\text{H}_2\text{O}$ , which is a colourless crystalline solid.

Crystals of zinc sulfate may be prepared in a school or college laboratory by reacting dilute sulfuric acid with a suitable compound of zinc.

Give the formulae of two simple compounds of zinc that could each react with dilute sulfuric acid to produce zinc sulfate.

(b)

A simple experiment to determine the value of $x$ in the formula $\text{ZnSO}_4\cdot x\text{H}_2\text{O}$ is to heat it carefully to drive off the water.

\text{ZnSO}_4\cdot x\text{H}_2\text{O(s)} \rightarrow \text{ZnSO}_4\text{(s)} + x\text{H}_2\text{O(g)}

A student placed a sample of the hydrated zinc sulfate in a weighed boiling tube and reweighed it. He then heated the tube for a short time, cooled it and reweighed it when cool. This process was repeated four times. The final results are shown below.

mass of empty tube /g	mass of tube + hydrated salt /g	mass of tube + salt after fourth heating /g
74.25	77.97	76.34

(i)

Why was the boiling tube heated, cooled and reweighed four times?

(ii)

Calculate the amount, in moles, of the anhydrous salt produced.

(iii)

Calculate the amount, in moles, of water driven off by heating.

(iv)

Use your results to (ii) and (iii) to calculate the value of $x$ in $\text{ZnSO}_4\cdot x\text{H}_2\text{O}$ .

(c)

For many people, an intake of approximately $15\text{ mg}$ per day of zinc will be sufficient to prevent deficiencies.

Zinc ethanoate crystals, $(\text{CH}_3\text{CO}_2)_2\text{Zn}\cdot 2\text{H}_2\text{O}$ , may be used in this way.

(i)

What mass of pure crystalline zinc ethanoate ( $M_r = 219.4$ ) will need to be taken to obtain a dose of $15\text{ mg}$ of zinc?

(ii)

If this dose is taken in solution as $5\text{ cm}^3$ of aqueous zinc ethanoate, what would be the concentration of the solution used?

Give your answer in $\text{mol dm}^{-3}$ .

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4 more questions

Q2Group 17 · Equilibria13M
Q3Nitrogen and Sulfur · Equilibria · Atoms, Molecules and Stoichiometry · States of Matter · Chemical Bonding13M
Q4Carbonyl Compounds · Hydroxy Compounds · Carboxylic Acids and Derivatives12M
Q5Hydroxy Compounds · Carboxylic Acids and Derivatives · Hydrocarbons · Introduction to Organic Chemistry9M

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