Question

How many grams of potassium chloride are needed to make 100 ml of a solution containing 250 mosmol/l? (m.w. of potassium is 39 and m.w. of chloride is 35.5). (answer must be numeric; no units or commas; include a leading zero when answer is less than 1; round the final answer to the nearest tenth.)?

Answer 1

In dilute solutions, the unit osmolarity is being used. It usually has units milliosmols per liter of solution or mOsmol/L. An osmole defines the number of moles of the solute that would have an effect on the osmotic pressure of the solution. Osmolarity is calculated by the product of the molarity and the number of particles in the solution which is 2 for potassium chloride. We calculate as follows:
Osmolarity = molarity (# of particles)250 mosmol/L ( 1 osmol / 1000 osmol) = x moles / .100 L (2)
x moles = 0.0125 mol KCl
mass KCl = 0.0125 mol KCl ( 39 + 35.5 g/mol) = 0.93125 g KCl