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Difficulty: MediumComposition and Pollution of Air

A 600 cm3600\text{ cm}^3 sample of air contaminated with sulphur(IV) oxide (SO2\text{SO}_2) gas was passed through an excess aqueous solution of sodium hydroxide to absorb all the SO2\text{SO}_2, reducing the volume of the gas sample to 576 cm3576\text{ cm}^3. The remaining gas mixture was then passed over excess heated copper turnings to remove oxygen gas, after which the unreacted gas volume measured 456 cm3456\text{ cm}^3 under the same conditions of temperature and pressure. What is the percentage by volume of sulphur(IV) oxide in the original air sample?

Answer: 4 %

Answer

The percentage by volume of sulphur(IV) oxide in the original air sample is 4.0%.
Sodium hydroxide (NaOH\text{NaOH}) selectively absorbs acidic pollutant gases such as sulphur(IV) oxide (SO2\text{SO}_2). The volume decrease from 600 cm3600\text{ cm}^3 to 576 cm3576\text{ cm}^3 indicates that 24 cm324\text{ cm}^3 of SO2\text{SO}_2 was absorbed. Dividing 24 cm324\text{ cm}^3 by the original sample volume of 600 cm3600\text{ cm}^3 and multiplying by 100 gives 4.0%4.0\%.

Step-by-Step Solution

1
Calculate the volume of sulphur(IV) oxide gas absorbed by the sodium hydroxide solution.
Volume of SO2=600 cm3−576 cm3=24 cm3\text{SO}_2 = 600\text{ cm}^3 - 576\text{ cm}^3 = 24\text{ cm}^3.
Sodium hydroxide reacts with acidic oxide pollutants like SO2\text{SO}_2, causing a reduction in gas volume equal to the volume of SO2\text{SO}_2 present.
2
Calculate the percentage composition by volume relative to the total initial sample.
\text{Percentage of } \text{SO}_2 = \left(\frac{24\text{ cm}^3}{600\text{ cm}^3}\right) \times 100\% = 4.0\%.
The volumetric percentage is determined by expressing the volume of the target gas component over the total volume of the original air sample.

Key Concept

Volumetric determination of air composition and gaseous pollutants.
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Topics

Composition and Pollution of Air

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