Question

Seawater contains 3.5 g of salts for every liter of solution.

Assuming that the solute consists entirely of NaCl (over 90% is), calculate the osmotic pressure (in atm) of seawater at 20*C.

Answer 1

The osmotic pressure of sea water can be calculated using Van’t Hoff Equation

π = iMRT

where π is the osmotic pressure

i is the van’t Hoff factor

M is Molarity

R is the contant – 0.08206 L atm mol^-1 K^-1

T is the temperature

First the temperature has to be converted to Kelvin

T = 20˚C = 20 + 273 = 293 K

Next the molarity

Molarity can be calculated by taking the amount of salt divided by the molecular weight of the salt

Molecular weight of NaCl = 58.44g/mol

Therefore

= 3.5g/58.44g/mol

= 0.0599 M

Van’t Hoff factor for NaCl – 2

Substituting all the values in the equation

π = 2 * 0.0599 * 0.08206* 293

π = 2.88 atm