Making predictions using the reactivity series

2 marks · Easy

When 1.0 g of an unknown metal (X) is added to excess dilute sulfuric acid at room temperature, 0.067 g of hydrogen gas is released. Given that the metal is above hydrogen in the reactivity series, predict whether magnesium or iron is more likely to be metal X. Show your reasoning.

Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.

Show answer

Magnesium is more likely to be metal X; the amount of hydrogen produced is closer to that expected from magnesium's reaction with sulfuric acid than iron's.

Must show: Comparing hydrogen yield for Mg and Fe, Identification of metal X as magnesium

Also accept: Calculation showing Mg produces similar amount of hydrogen gas as observed

Common mistakes: Assuming reactivity is unrelated to the gas produced; Ignoring molar ratios

Show mark scheme and worked solution

Marking points

M1 · 1 mark · Correct method/calculation: Shows working to compare hydrogen production from 1.0 g of Mg and Fe

A1 · 1 mark · Correct conclusion: Magnesium is the most likely identity for X

Worked solution

The reaction for magnesium:

Molar mass of Mg = 24 g/mol. 1 mol Mg gives 1 mol H2 (2 g).

Hydrogen from 1.0 g Mg: mol Mg = 0.0417 mol, so 0.0417 mol H2 = 0.083 g.

Theoretical yield for iron:

Molar mass Fe = 56 g/mol. 1.0 g Fe = 0.0179 mol, so H2 = 0.0358 g.

Observation (0.067 g) is much closer to magnesium's yield (0.083 g) than to iron (0.036 g), so X is more likely magnesium.