Given that you only know the mass in grams of the solute and the mass in grams of the solution, you can only calculate the concentration as percent mass/mass.
The formula is:
Percent m/m, % = [ (mass in grams of solute) / (mass in grams of solution) ] * 100
% = [ (1.2 * 10^ -3 g) / (800 g) ] * 100 = 0.0015%
Answer: 0.0015%
