Question

What is the osmolarity of distilled water? what is the osmolarity of 40mM CaCl2?

Is it hypertonic, isotonic or hypotonic?

Answer 1

Ans. Osmolarity is the number of osmoles of solute per liter of solution.

Osmole is the number of moles of a solute that contributes to osmolarity of the solution.

For an electrolyte (produces two or more ions in aqueous solution), number of osmoles is equal to number of ions produced per mol electrolyte.

For non-electrolytes like sucrose, van’t Hoff factor, i=1.

So,

Osmolarity = (i x Molarity) osmole/L ;           [where, i = Van’t Hoff factor]

# Given,          [CaCl₂] = 40.0 mM = 0.040 M                     ; [1 mM = 0.001 M]

CaCl₂ dissociates to form 3 ions = 1 Ca²⁺ ions + 2 Cl^-1 ions.

So, i = 3 for CaCl₂

Now,

            Osmolarity of 40 mM CaCl₂ = (3 x 0.040) osmol/L = 0.120 osmol/ L

# Assuming absence of any type of solute in distilled water, the osmolarity of distilled water is taken to be 0.0 (zero).

# Since osmolarity of 40 mM CaCl₂ is greater than that of distilled water, the CaCl₂ solution is said to be HYPERTONIC with respect to distilled water.

Or,

Distilled water is said to be HYPOTONIC with respect to CaCl₂ solution because the osmolarity of distilled water (= 0.0) is less than that of CaCl₂ solution (= 0.120).

# Two solutions are said to be ISOTONIC when their osmolarity is equal to each other.