molarity, molality

[sugars225]

concentrated nitric acid solution has a density of  1.42g/mL, and is 69.0% HNO3 by mass

a) determine the molarity of the solution with respect to HNO3
b) molality of the solution with respect to HNO3

I calcualated the molarity to be  15.6mol/L but I do not get how to do b

[freezard7734]

Molality is simply the number of moles of solute divided by the number of kilograms of solvent.
To make life simpler, assume we are working with 1 L of solution. Then there are 1.42 kg of solution. 69% of that is HNO3, so from that you can get the number of moles of HNO3. Then 31% is water (solvent).
So molality is (moles of HNO3) / (.31 * 1.42 kg H2O) = 35.3 molal

[sugars225]

Thank you very much