24g of an unknown gas occupied 8.4dm3at STP, Identify the gas among the following
Correct answer: B. SO2
- A. SO3
- B. SO2
- C. C6H6
- D. C3H8
Explanation
At STP, 1 mol of gas occupies 22.4 dm3. The amount of gas is 8.4 dm3/22.4 dm3 mol-1 = 0.375 mol, so its molar mass is 24 g/0.375 mol = 64 g mol-1, which matches SO2. Students may choose SO3 because it contains the same elements, but SO3 has molar mass 80 g mol-1.
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About Standard Temperature and Pressure
Standard temperature and pressure provide reference conditions for gas calculations: 0°C or 273 K and, in the usual school convention, 1 atm pressure. Questions use these conditions to relate pressure, volume, temperature and amount of gas, including the molar volume of an ideal gas, about 22.4 dm³.
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