Question

What is the molarity of a solution of KCL (KCL, PF=58.5) in water (H2O, PM=18) when the fraction molar of KCL is 0.0390? The density of the solution is 1105g/lL

Answer 1

consider 1 mol of solution
then mol of KCl = 0.0390
mol of H2O = 1 - 0.0390 = 0.961
Now use:
mass of H2O = mol*molar mass of H2O
= 0.961 mol * 18 g/mol
= 17.3 g

mass of KCl = moles of KCl * molar mass
= 0.0390 mol * 58.5 g/mol
= 2.282 g

mass of solution = 17.3 g + 2.282 g = 19.582 g

now use:
density = mass / volume
1.105 g/mL = 19.582 g / volume
volume = 17.72 mL
volume = 0.01772 L

use:
Molarity = moles of KCl / volume of solution
= 0.0390 mol/0.01772 L
= 2.20 M

Answer: 2.20 M