Question

The total concentration of dissolved ions in seawater is about 1.1 M, but concentrations can vary depending on the exact location and conditions. What minimum pressure must be supplied to a reverse osmosis water purification system in order to overcome the osmotic pressure in water that is 1.06 M in total dissolved ions at 34 C?

Answer 1

The osmotic pressure in seawater with 1.06 M of total ions concentration would be calculated using vant Hoff expression:

$p=iCRT,$

where i is equal to $1-a(n-1)$.

As for the seawater i or vant Hoff factor is nearly equal to the number of ions produced by one formula unit of salt, like NaCl etc. Thus, i $\geq$ 2 (NaCl = Na⁺ + Cl^-, two ions).

Now it is the right time to calculate the minimum pressure required:

$p \geq 2*1060 \frac{mol}{m^3}*8.31 \frac{J}{mol*K}307K=2.7MPa$.