Question

Magnesium metal reacts with phosphoric acid to produce hydrogen gas. If 50 g of phosphoric acid is used, how much magnesium metal is needed? How much hydrogen gas is produced?

Answer 1

3Mg + 2H3PO4 ==> 3H2 + Mg3(PO4)2

mols H3PO4 = grams/molar mass.
Using the coefficients in the balanced equation, convert mols H3PO4 to mols Mg.

Then convert mols Mg to grams. g = mols Mg x atomic mass Mg.