Calculate the molarity of 0.850 mol of Na2S in 1.70 L of solution.

Molarity is simply the ratio of the number of moles of a substance over the total volume of the solution. Assuming that the addition of 0.850 moles does not change the overall volume of 1.70 L, therefore molarity is simply:

Molarity = 0.850 moles / 1.70 L = 0.5 moles / L = 0.5 M