Question

Calculate the external pressure that must be applied to seawater, 1.14 M total ion concentration at 10 degrees C if the maximum concentration allowed in the product water is 176 mg/L. Assume that all the dissolved salts in the product water is sodium chloride.
So, I know that I need to subtract the ion concentrations before using the pi=MRT formula, but I can't figure out how to convert mg/L into molarity. Please help!

Answer 1

C = 1.14 M , T = 10 + 273.15 = 283.15 K   salt concentration = 176 mg /L NaCl
π total = CRT = 1.14 x 0.0821 x 283.15 = 26.5 atm
mass of NaCl = 176 mg = 0.176 g
Molarity = $\frac{0.176}{57.96} x \frac{1}{1L} = 3.0366 x 10^{-3} M$
maximum concentration allowed C = 3.0366 x 10⁻³ M
π max. conc. = 2 x 3.0366 x 10⁻³ x 0.0821 x 283.15 = 0.141 atm
π external = π total - π max. conc.
                 = 26.5 - 0.141 = 26.3 atm