If the density of ocean water at a depth of 1.00 x 104 m is 1.071 g/ml and if 21.0 g of water at that depth contains 197 mg of potassium chloride, what is the molarity of potassium chloride in the sample

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