The molarity concentration is simply the ratio of the
number of moles of solute over the total volume of the solution.
The molar mass of potassium chloride is 74.55 g/mol, so
the moles is:
moles KCl = 0.197 g / (74.55 g/mol) = 2.64 x 10^-3 mol
Assuming that the volume of KCl is negligible so that the
total volume of solution is simply equal to the volume of water, so:
Volume = 21 g / [(1.071 g / mL) * (1000 mL / 1 L)] = 0.0196
L
So the molarity is:
Molarity = 2.64 x 10^-3 mol / 0.0196 L
Molarity = 0.135 moles / L = 0.135 M
