Calculate the mass of the salt that forms when 4 g of magnesium oxide reacts with sulfuric acid.

You should, first, write the reaction equation:

MgO + H2SO4 = MgSO4 + H2O (do not forget to equalize if the reaction requires it);

We are given the mass of magnesium oxide, so we find through it the number of moles (n) of magnesium oxide itself:

n (MgO) = m (MgO): M (MgO) = 4 g: 40 g / mol = 0.1 mol;

All substances in the equation have an equal number of moles, since all of them are preceded by a coefficient 1. It follows that n (MgSO4) = n (MgO) = 0.1 mol;

It remains only to find the mass of our salt:

m (MgSO4) = n (MgSO4) * M (MgSO4) = 0.1 mol * 120 g / mol = 12 g.