Question

If 4.50 g of (nh4)2so4 is dissolved in enough water to form 250. ml of solution, what is the molarity of the solution?

Answer 1

The molarity of a solution is the number of moles of a substance in one liter of that substance. 
The molar mass of ammonium sulfate (NH4)2SO4 is 132.14 grams/mole
Calculate the moles of ammonium sulfate:
(4.50 grams)/(132.14 grams/mole) = 0.0341 moles of ammonium sulfate
convert mL to Liters 250. mL becomes 0.250 liters
Take the number of moles over the number of liters
0.0341 moles / 0.250 liters = 0.136 molar or 0.136M = molarity of the solution