Consider the equation H2 + O2 gives H2O. What volume of hydrogen gas is required to produce 1 mol of water at standard temperature and pressure?
Correct answer: B. 22.4 dm3
- A. 11.2 dm3
- B. 22.4 dm3
- C. 10 dm3
- D. 58 dm3
Explanation
Balanced properly the equation is 2H2 plus O2 giving 2H2O, so one mole of water needs one mole of hydrogen, and one mole of any gas occupies 22.4 dm3 at STP. The figure 11.2 dm3 is half a mole and is the trap for anyone who halves the ratio the wrong way. Molar volume is independent of the identity of the gas, which is Avogadro's law.
This question appeared on the UHS MDCAT 2025 paper, which you can sit online with every answer explained.
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About Moles and Avogadro's Number
The mole connects the microscopic number of particles with measurable mass, using Avogadro's number, 6.022 × 10²³ particles per mole. Questions involve molar mass, conversion between mass, moles and particles, percentage composition, empirical and molecular formulas, and distinguishing atoms, molecules, ions and formula units.
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