calculate the osmotic pressure of river water at 20.0 degree Celsius

Question

A sample of river water is found to contain 2.95 g of salt for every liter of solution. Assuming that the salt consists entirely of NaCl, calculate the osmotic pressure of river water at 20 degree Celsius.

Answer 1

Use the osmotic pressure formula: π = iC RT where π is the osmotic pressure in atmospheres, C the molar concentration, R the gas constant, T the absolute temperature, i the van't Hoff factor.

In the case of a dilute solution, NaCl dissociates into two ions per formula unit, so the van't Hoff factor ideally is 2. The actual factor is dependent on the concentration, being lower than 2 for more concentrated solutions. Let's assume a factor of 2.

π = 2 x 2.95g /L / 58.44 g/mole x 0.0821 L-atm/mol-deg K x 293.15 deg K = 2.43 atm