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Hydrogen mass for ammonia
The equation for the reaction between nitrogen and hydrogen to produce ammonia is:
Relative atomic masses (Aᵣ): H = 1 N = 14
Calculate the mass of hydrogen that is needed to produce 25 g of ammonia.
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Marking points
- 1 Uses a valid relative-mass and stoichiometric route based on the balanced equation.
- 2 Gives a final hydrogen mass of 4.41 g to at least 2 significant figures.
Full-mark answer
Mᵣ(NH₃) = 17, so moles of NH₃ = 25 ÷ 17 = 1.4706 mol. From N₂ + 3H₂ → 2NH₃, moles of H₂ = 1.4706 × 3 ÷ 2 = 2.2059 mol. Mass of H₂ = 2.2059 × 2 = 4.41 g.
Why this answer loses marks
I found the moles of ammonia and used the same number of moles for hydrogen.
This skips the balanced-equation ratio, which must be applied before converting the target moles into a mass.