Question

what is the % (m/m) of Mg(NO3)2 in solution when 0.84 g of Mg(NO3)2 are dissolved in enough deionized water to prepare 500.0 ml of solution ( assume the solution in dilute enough that the density of the solution is 1.0 g/ml)

Answer 1

% m/m = grams solute/g solution.
You have 0.84 g solute and 500 solution.(density soln is 1.0 g/ml. You have 500 mL and 500 mL x 1.0 g/mL = 500 grams).