Here's how to determine the molecular formula of ammonium sulfate using the criss-cross method:

1. Identify the Ions:

* Ammonium: NH₄⁺ (This is a polyatomic ion with a +1 charge)

* Sulfate: SO₄²⁻ (This is a polyatomic ion with a -2 charge)

2. Criss-Cross the Charges:

* The ammonium ion (NH₄⁺) has a +1 charge, so we write a subscript of 2 for the sulfate ion.

* The sulfate ion (SO₄²⁻) has a -2 charge, so we write a subscript of 1 for the ammonium ion (which is understood and not written).

3. Write the Formula:

This gives us the formula: (NH₄)₂SO₄