A repair workshop has collected filings from brass fittings. Brass is an alloy of copper and zinc. To estimate the zinc content, a chemist treats separate s...
Question 1 Report
A repair workshop has collected filings from brass fittings. Brass is an alloy of copper and zinc. To estimate the zinc content, a chemist treats separate samples with excess dilute hydrochloric acid. Zinc reacts to release hydrogen, whereas copper does not react with this acid. Fig. 1 shows one sample in a flask connected to a gas syringe. After each test, the remaining copper is filtered, washed with water, dried and weighed.
| Mass of alloy sample / g | Hydrogen volume at RTP / cm3 | Mass of dry residue / g |
|---|---|---|
| 1.95 | 480 | 0.65 |
| 3.90 | 960 | 1.30 |
Zn + 2HCl → ZnCl2 + H2
Use 24 000 cm3 mol-1 for the molar gas volume and 65.0 g mol-1 for zinc. The acid is deliberately used in excess.
(a) Give the identity of the dry residue. [1]
(b) Use the first result to calculate the amount, in mol, of hydrogen produced. [2]
(c) Use the equation to calculate the mass of zinc in the first sample. [3]
(d) Use the data to calculate the percentage by mass of zinc in the alloy. [2]
(e) Describe two steps needed before weighing the copper residue. [2]
(f) Give two observations expected while zinc reacts with hydrochloric acid. [2]
Answer Details
- The dry residue is copper, because copper does not react with dilute hydrochloric acid. [1]
- \[n(\mathrm{H_2})=\frac{480\text{ cm}^3}{24000\text{ cm}^3\text{ mol}^{-1}}=0.0200\text{ mol}\]
[2] - The equation shows \(\mathrm{Zn:H_2}=1:1\), so \(n(\mathrm{Zn})=0.0200\text{ mol}\).
\[m(\mathrm{Zn})=0.0200\times65.0=1.30\text{ g}\]
[3] - \[\%\mathrm{Zn}=\frac{1.30}{1.95}\times100=66.7\%\]
Therefore the alloy is 66.7% zinc, or 67%. [2] - Filter the mixture to separate the copper, wash the residue to remove acid and zinc chloride solution, then dry it before weighing. Cooling is also needed before weighing if it has been heated. Any two steps gain credit. [2]
- Expected observations include bubbles or effervescence, zinc filings becoming smaller or disappearing, warming of the flask, and collection of a colourless gas. Any two gain credit. [2]
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