How to calculate molality with density and molarity?

Molarity and Molality help?

• B is a part of a, which is what I really need help on. a) A solution of phosphoric acid was made by dissolving 10.5 g of H3PO4 in 100.0mL of water. The resulting volume was 104mL. Calculate the molarity of this solution. 1.03M b) Assuming the density of water is 1.00g/mL, calculate the molality of the solution. Please help, I've been stuck on this for a while.

• Answer:

  hi!! a) 1.03 you are correct there........ :) b) molality = amount of solute / mass of solvent molality = moles of solute / kg of solvent moles of H3PO4 = mass / molar mass moles = 10.5 / 98 moles = 0.107 moles The total volume of the solution is 0.104L (= 104 ml) The density of the solution is 1g/ml So the total mass of the solution is 1g/ml x 104ml = 104g mass of solvent = total mass - mass of solute = 104 - 10.5 = 93.5g = 0.0935kg therefore molality = moles solute/ kg solvent = 0.107mols / 0.0935kg = 1.144mols/kg = 1.144 molal so there u go........... i hope now u wil nt get stuck hope u understood....... :) ^_^