Question

Formula for calculating percentage molalty of a solution when % mass is given

Answer 1

Explanation:

Your goal here is to figure out the number of grams of solute present for every 100 g of the solution, i.e. the solution's percent concentrationby mass, % m/m.

Now, you know that the solution has a molalityequal to 2.35 mol kg−1. This tells you that this solution contains 2.35 moles of rubidium nitrate, the solute, for every 1 kg of water, the solvent.

To make the calculations easier, pick a sample of this solution that contains exactly 1 kg=103  gof water. As we've said, this sample will also contain 2.35 moles of rubidium nitrate.

Use the molar mass of the solute to convert the number of moles to grams

2.35moles RbNO3⋅147.473 g1mole RbNO3=346.56 g

This means that the total mass of the sample is equal to

346.56 g +  103  g=1346.56 g

So, you know that you have 346.56 g of rubidium nitrate in 1346.56 g of the solution, so you can say that 100 g of this solution will contain

100g solution⋅346.56 g RbNO31346.56g solution=25.7 g RbNO3

This means that the solution's percent concentration by mass is equal to

% m/m=25.7% RbNO3−−−−−−−−−−−−−−−−−−−−−

This tells you that you get 25.7 g of rubidium nitrate for every 100 g of the solution.