Osmolarity Calculator

Calculate osmolarity (Osm/L) from solute concentration and number of dissociating particles. Compare to blood osmolarity (290 mOsm/L). Used for IV fluid preparation and cell biology.

💧 Solutions📐 Osm = C × i🧪 Chemistry
Concentration C (mol/L)
van't Hoff factor i
Additional solute C2 (mol/L)
i2 for second solute
Please enter valid values.

Formula & Reference

VariableSymbolFormulaUnits
Osmolarity Calculator—Osm = C × iOsm/L or mOsm/L

Step-by-Step Examples

Example 1
Normal Saline 0.9% NaCl

0.154 M NaCl (i=2). Standard IV solution.

  • Osm = 0.154 x 2 x 1000 = 308 mOsm/L
  • Slightly hypertonic vs blood (290 mOsm/L)
  • But physiologically equivalent to blood
✓ 308 mOsm/L (approximately isotonic)
Example 2
5% Dextrose in Water (D5W)

5.0 g/100 mL glucose = 0.278 M (M=180.16). i=1.

  • Osm = 0.278 x 1 x 1000 = 278 mOsm/L
  • Slightly hypotonic
  • After glucose metabolism: leaves free water (hypotonic effect)
✓ 278 mOsm/L D5W
Example 3
Ringer Lactate

Na+: 130 mM, K+: 4 mM, Ca^2+: 1.5 mM, Cl^-: 109 mM, lactate: 28 mM.

  • Osm = (0.130+0.004+0.0015+0.109+0.028) x 1000 (each ion counts separately)
  • = 0.2725 M x 1000 (approx since already ions)
  • Total ~ 273 mOsm/L
✓ ~273 mOsm/L (slightly hypotonic, used for fluid resuscitation)

Real-World Applications

🏥
IV Fluid Design
Isotonic saline (308 mOsm/L) chosen to prevent cell swelling or shrinking.
🧬
Cell Biology
Osmolarity of cell culture media must match cell osmolarity (~290 mOsm/L) to prevent osmotic stress.
💊
Pharmacy
Ophthalmic drops must be isotonic (~300 mOsm/L) to prevent eye irritation.
⚗️
Nephrology
Kidney concentrates urine up to 1200 mOsm/L. Urine osmolarity assesses kidney function.

Common Mistakes to Avoid

⚠️
Osmolarity vs Osmolality

Osmolarity = Osm/L solution. Osmolality = Osm/kg solvent. For dilute aqueous: nearly equal. Osmolality used for clinical measurements (freezing point depression).

⚠️
Each dissociated ion counts separately

NaCl: Na+ + Cl- = 2 osmoles per mole. CaCl2: Ca^2+ + 2Cl- = 3 osmoles per mole. MgSO4 (non-dissociated pair): 1.

⚠️
Blood osmolarity is ~290 mOsm/L

Na+ (~140) + Cl- (~103) + HCO3- (~24) + glucose (~5) + BUN (~5) + K+ (~4) = ~281 mOsm/L blood.

Frequently Asked Questions

What is tonicity? ▾
Tonicity describes effect on cells: isotonic (no change), hypotonic (cells swell), hypertonic (cells shrink). Depends on permeable vs non-permeable solutes.
What is the difference between osmolarity and tonicity? ▾
Osmolarity = total solute concentration. Tonicity = effective osmolarity (only non-penetrating solutes). Urea contributes to osmolarity but not tonicity (enters cells freely).
What happens when cells are in hypotonic solution? ▾
Water enters cells by osmosis (solute concentration inside > outside). Cells swell and may lyse. Red blood cells in pure water burst (hemolysis).
How does the kidney control osmolarity? ▾
ADH (antidiuretic hormone) makes collecting ducts permeable to water. High ADH: concentrated urine (1200 mOsm/L). No ADH: dilute urine (50 mOsm/L).
Why is normal saline slightly hypertonic? ▾
0.9% NaCl = 308 mOsm/L. Blood = 290 mOsm/L. Small difference is clinically acceptable and avoids hemolysis. Isotonic to red blood cells in practice.

Related Chemistry Calculators

💧 Osmotic Pressure🌡️ Boiling Point Elevation🧪 Molarity🧪 Solution Prep⚗️ Mole Fraction

Formula Explorer connections

Interpretation: This formula connects solute amount and solution volume, mass or particle count to concentration and colligative behavior. Assumption: Distinguish solution volume from solvent volume, use the stated temperature, and account for dissociation or nonideal activity when required.

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