We first need to convert grams of ammonium sulfate to moles.
molarity = moles of solute/liters of solution
To convert from grams to moles, we need to divide by it's molecular mass.
26.42 g (NH₄)₂SO₄ * (1 mole (NH₄)₂SO₄ / 132.14 g (NH₄)₂SO₄) = 0.1999 mole (NH₄)₂SO₄
0.1999 mol/0.05000L = 3.998 M
Since there 4 significant digits in both the volume and mass, we are limited to 4 significant digits.
Your final answer is 3.998 M (NH₄)₂SO₄.
