When 0.07 moles of ammonium sulfate ((NH4)2SO4) are dissolved in enough water to make 254 milliliters of solution, how many ammonium ions are present?

Answer 1

You have 0.07 mols (NH4)2SO4.
There are 6.02E23 molecules in 1 mol (NH4)2SO4 so there are
6.02E23 x 0.07 = ? molecules (NH4)2SO4 in that 0.07 mols.
Since there are two NH4^+ in 1 molecule of (NH4)2SO4, there must be twice the number of NH4^+ ions as there are (NH4)2SO4 molecules.