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.
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