Calculate the mass of concentrated sulfuric acid required to dissolve 20 g of magnesium, containing 10% of impurities.

Given:

m (Mg) = 20 g

w% (impurities) = 10%

To find:

m (H2SO4) -?

Decision:

H2SO4 + Mg = MgSO4 + H2, – we solve the problem based on the composed reaction equation:

1) Find the mass of pure magnesium that has reacted:

If the proportion of impurities is 10%, then the proportion of pure magnesium = 90%, then

m (Mg) = 20 g * 0.9 = 18 g

2) Find the amount of magnesium:

n (Mg) = m: M = 18 g: 24 g / mol = 0.75 mol

3) We compose a logical expression:

If 1 mole of Mg requires 1 mole of H2SO4,

then 0.75 mol Mg will require x mol H2SO4,

then x = 0.75 mol.

4) Find the mass of sulfuric acid required for the reaction to proceed:

m (H2SO4) = n * M = 0.75 mol * 98 g / mol = 73.5 g.

Answer: m (H2SO4) = 73.5 g.