Question

What is the mass percent concentration of the following solutions? (a) Dissolve 0.655 mol of citric acid, C6H8O7, in 1.00 kg of water. (b) Dissolve 0.135 mg of KBr in 5.00 mL of water. (c) Dissolve 5.50 g of aspirin, C9H8O4, in 145 g of dichloromethane, CH2Cl2.

Answer 1

A)

MW (citric acid): 192.1235. So, the mass of citric acid would be 0.655(192.1235) = 125.₈ g. Obviously, the mass of the solution would the be 1.12₅₈ kg and the mass percent would be just

B)

Assume 1 mL of water to be 1 g. Then, 5.00 mL = 5000 mg and

for the mass-% of KBr. Note that we were a little sloppy with sig. figs. in the intermediate steps but did the right thing at the end!

C)

This is quite simple in that we don't have to fiddle with units! Anyway,

It's nice when the last part of a problem is the easiest!