What is the molarity of this ion?

Question

The average concentration of potassium ion in seawater is 400. mg of potassium ion per kg of seawater. What is the molarity of the potassium ion if the density of the seawater is 1.025 g/mL.

Answer 1

Molarity is the number of moles of a solute per liter (1000 ml) of solution. We know the concentration in grams of K per kg of solution. (Of course, 400 mg K = 0.4 gm K)

First, use the density of the solution to convert the known concentration to grams K per liter of seawater:

(1.025 kg seawater)/(liter seawater) * (0.4 gm K)/(kg seawater)
= (0.41 gm K)/(liter seawater)

The atomic mass of K is 39 .0983 gm/mole. Use this to calculate how many moles of K are in 0.41 gm.

molarity = (0.41 gm K)/(39.0983 gm K/mol K)/(liter seawater)
molarity = (1.049 * 10^-2 mol K)/(liter seawater)