When 5.0 g of hydrogen gas reacts with excess oxygen at 25°C, how many grams of water are formed? (Molar mass $H_{2} = 2 g mol^{- 1}$ , $O_{2} = 32 g mol^{- 1}$ , $H_{2} O = 18 g mol^{- 1}$ )

medium 3 marks Class 10 · Science · Chemical Reactions and Equations

Answer

45 g of water

The correct answer is 45 g of water — Convert moles of water to mass: $2.5 mol \times 18 g mol^{- 1} = 45 g$ .

Step-by-step solution

Write the balanced equation: $2 H_{2} + O_{2} \to 2 H_{2} O$ .
Convert the given mass of $H_{2}$ to moles: $\frac{5.0 g}{2 g mol ^{- 1}} = 2.5 mol$ .
From the equation, 2 mol $H_{2}$ produce 2 mol $H_{2} O$ ; the mole ratio $H_{2} : H_{2} O$ is 1:1.
Therefore, $2.5 mol$ of $H_{2}$ will give $2.5 mol$ of $H_{2} O$ .
Convert moles of water to mass: $2.5 mol \times 18 g mol^{- 1} = 45 g$ .

Common mistake: Using an incorrect mole ratio between

H_{2}

and

H_{2} O

Concept tested: Stoichiometry using balanced equations