A chemist uses hot hydrogen gas to convert chromium(III) oxide to pure chromium. How many grams of hydrogen are needed to convert 76 grams of chromium(III) oxide, Cr203?
Cr2O3 + H2Cr + H2O

3.0242 g of H2

Explanation:

The balanced equation for the reaction is given as;

Cr2O3 + 3H2 → 2Cr + 3H2O  

From the reaction, the mole ratio between Cr2O3 and H2 is given as;

1 mol of Cr2O3 reacts with 3 mol of H2

Converting to mass using;

Mass = Molar mass * Number of moles

Cr2O3;

Mass = 151.99 g/mol * 1 mol = 151.99 g

H2;

Mass = 2.016 g/mol * 3 mol = 6.048 g

How many grams of hydrogen are needed to convert 76  grams of chromium(III) oxide, Cr203?

151.99 = 6.048

76 = x

x = 76 * 6.048 / 151.99

x = 3.0242 g