The table below shows results from a recycling plant that recovers zinc from discarded alkaline cells. The cells contain zinc and potassium hydroxide soluti...
Question 1 Report
The table below shows results from a recycling plant that recovers zinc from discarded alkaline cells. The cells contain zinc and potassium hydroxide solution. After washing, one zinc sample is heated in a furnace with a measured supply of oxygen. Fig. 1 shows the covered crucible used for the heating stage. The lid reduces loss of solid powder while allowing oxygen to enter. The plant compares the mass of zinc with the mass of the zinc oxide product to check whether the product has the expected composition.
| Material | Mass / g |
|---|---|
| Zinc placed in crucible | 6.54 |
| Zinc oxide after heating | 8.14 |
The relative atomic masses are Zn = 65.4 and O = 16.0. A second sample may contain soluble sodium chloride from the cell-processing equipment. The quality-control chemist tests the water extract rather than adding acid directly to the zinc oxide.
(a) Use Table 1 to calculate the mass of oxygen that reacted with the zinc. [2]
(b) Use the data and the relative atomic masses to calculate the empirical formula of the oxide. Show your working. [4]
(c) Explain why the mass of zinc oxide is greater than the mass of zinc, without contradicting conservation of mass. [3]
(d) Describe a test for chloride ions in the water extract of the recycled material, including the positive result. [3]
Answer Details
(a) Oxygen mass is the product mass minus zinc mass:
\[8.14\text{ g} - 6.54\text{ g} = 1.60\text{ g}\]
1.60 g of oxygen reacted. [2]
(b) Calculate moles, then find the simplest ratio:
\[n(\mathrm{Zn})=\frac{6.54}{65.4}=0.100\text{ mol}\]
\[n(\mathrm{O})=\frac{1.60}{16.0}=0.100\text{ mol}\]
Zn : O = \(0.100:0.100=1:1\), so the empirical formula is \(\mathrm{ZnO}\). [4]
(c) Zinc oxide has a greater mass because oxygen atoms from the gas combine with zinc atoms. The 1.60 g of oxygen has been added to the zinc. Conservation of mass still applies when zinc, oxygen and zinc oxide are all included in the system. [3]
(d) Make a water extract and filter it. Add dilute nitric acid, then silver nitrate solution to the filtrate. A white precipitate shows chloride ions are present. [3]
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