Question

Seawater has a salinity of 3.5%, meaning that if you boil away a kilogram of seawater, when you're finished you'll have 35 g of solids (mostly NaCl) left in the pot. When dissolved, sodium chloride dissociates into separate Na⁺ and CL⁻ ions. (a) Calculate the osmotic pressure difference between seawater and fresh water. Assume for simplicity that all the dissolved salts in seawater are NaCl.

Answer 1

The osmotic pressure difference between seawater and freshwater can be calculated based on the concentration of sodium chloride (NaCl) in seawater. Using the equation II = MRT, the osmotic pressure difference is found to be 34 atm.

Step-by-step explanation:

The osmotic pressure difference between seawater and freshwater can be calculated based on the concentration of sodium chloride (NaCl) in seawater. Since seawater has a salinity of 3.5%, which means 35 g of NaCl is left when boiling a kilogram of seawater, we can assume all dissolved salts in seawater are NaCl for simplicity. Using the equation II = MRT, where M is the molar concentration, R is the ideal gas constant, and T is the temperature, we calculate the osmotic pressure difference to be 34 atm.

Answer 2

The osmotic pressure difference between sea water and fresh water is 29.3 atm

How to calculate the osmotic pressure difference?

First, we shall calculate the mole of the salt (i.e NaCl) in the fresh water. Details below:

• Mass of NaCl = 35 g
• Molar mass of NaCl = 58.5 g/mol
• Mole of NaCl =?

Mole of NaCl = Mass of NaCl / Molar mass of NaCl

= 35 / 58.5

= 0.598 mole

Next, we shall obtain the molarity of the salt. Details below:

• Mole of NaCl = 0.598 mole
• Mass of water = 1 kg = 1000 g
• Volume of water = Mass of water = 1000 = 1000 mL = 1 L
• Molarity of salt =?

Molarity of salt = mole / volume

= 0.598 / 1

0.598 M

Finally, we shall calculate the osmotic pressure. This is shown below:

• Molarity of salt (M) = 0.596 M
• Van't Hoff factor for salt, NaCl (i) = 2 (since NaCl is an ionic compound)
• Gas constant (R) = 0.0821 atm.L/Kmol
• Temperature = 295 K (i.e room temperature)
• Osmotic pressure (π) =?

π =iMRT

= 2 × 0.596 × 0.0821 × 295

= 29.3 atm