Question

how many moles of hydrogen will be liberated from 200 ml of 3 m hcl reacting with an excess of magnesium

Answer 1

Moles of HCl = 0.2L *3.0M = 0.6moles
From the reaction we can say that when 2 moles HCl reactcts it produced 1 mole of H2 gas
so, 0.6 moles of HCl * 1 mole of H2 / 2 moles of HCl = 0.3 moles of H2