The milliosmole (mOsmol) is the unit of measure for osmotic concentration. For nonelectrolytes, 1 millimole represents 1 mOsmol. However, for electrolytes, the total number of particles in a solution is determined by the number of particles produced in a solution and influenced by the degree of dissociation.

The ideal osmolar concentration can be calculated with the
following equation:

mOsmol= [𝑤𝑒𝑖𝑔ℎ𝑡 (𝑔) ÷ mol. wt.] × number of species × 1000

It can also be represented as mOsmol/L and gram/L

mOsmol/L = [𝑤𝑒𝑖𝑔ℎ𝑡 (𝑔/𝐿) ÷ mol. wt.] × number of species × 1000

**Calculation Example**

**1. What is the osmolarity of 0.9%w/v NaCl injection?**

Here,

·
0.9% w/v= 0.9 g/ 100 mL = 9 g/L

·
Molecular weight of NaCl = 58.5

mOsmol= [𝑤𝑒𝑖𝑔ℎ𝑡
(𝑔) ÷ mol. wt.] × number of species × 1000 = (9 ÷ 58.5) × 2 ×1000 = 308
mOsmol = 0.308 Osmol

**2. A solution contains 156 mg of K+ ions per 100 mL. How
many mOsmoles are represented in a liter of the solution? Atomic weight of K+
=39**

Here,

·
156 mg/100 mL = 0.156 g/ 100 mL= 1.56 g/L

mOsmol/L = No. of moles x No. of species x 1000

Number of moles= weight/ mol. wt. = 1.56/39= 0.04

mOsmol/L = 0.04 x 1 x 1000= 40 mOsmol/L.

**3. A parenteral solution contains 5% dextrose. How many
mOsmoles per liter are represented by this concentration?**

Here,

5% = 5 g/100 mL = 50 g/ L

Molecular weight (MW) of dextrose = 180

Dextrose is non-electrolyte, so, number of species = 1

mOsmol/L= [𝑤𝑒𝑖𝑔ℎ𝑡
(𝑔/𝐿)
÷
𝑀𝑊]
×
number of species X 1000

mOsmol/L = (50 ÷ 180) × 1 × 1000 = 278 mOsmol/L.

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