Answer
$(NH_{4})_{2}SO_{4}$
Work Step by Step
Ammonium ($NH_{4}$), as the problem states, has a charge of +1 while Sulfate ($SO_{4}$) has a charge of -2. Because all ionic compounds must have a neutral charge, there must be two Ammonium ions to cancel out the charge of the Sulfate ion, thus making the ionic compound $(NH_{4})_{2}SO_{4}$.
