Practice Problem 4

A saturated solution of hydrogen sulfide in water can be prepared by bubbling H2S gas into water until no more dissolves. Calculate the molality of this solution if 0.385 grams of H2S gas dissolve in 100 grams of water at 20oC and 1 atm.


The key to solving this problem is recognizing that we can only calculate the molality of a solution if we simultaneously know the number of moles of solute and the number of kilograms of solvent used to prepare the solution.

The number of moles of solute in this solution is easy to obtain.

Now all we have to do is divide the number of moles of solute by the mass of the solvent in kilograms to find that this solution is 0.113 molal.

0.0113 mol H2S = 0.113 m  
0.100 kg H2O