Question

What mass of manganese(II) chloride must react with sulfuric acid to release 48.5 mL of hydrogen chloride gas at STP? MnCl2(s)+H2SO4(aq)→MnSO4(aq)+2HCl(g)

Answer 1

Note that one mole of any gas at STP occupies 22.4 L of volume. We may use this to calculate the moles of hydrogen chloride gas that need to be produced. This is:
0.0485 / 22.4
0.002 mol

Next, we see that the molar ratio between manganese chloride and hydrogen chloride in the equation is:
1 : 2

So the moles of manganese chloride reacting will be:
0.002 / 2 = 0.001 mol

The molecular mass of manganese chloride is 126 g/mol. The mass reacting will be:
0.001 * 126

0.126 grams of manganese (II) chloride will react.