Osmolarity Calculator

Molarity counts formula units dissolved; osmolarity counts the particles they become. Sodium chloride splits into two ions, so 0.154 M NaCl is about 308 mOsm/L rather than 154.

The number of particles one formula unit produces in solution: 1 for glucose, 2 for NaCl, 3 for CaCl₂. Measured values fall slightly short of the whole number, because some ions pair up rather than staying independent.

Osmolarity is per litre of solution, osmolality per kilogram of solvent. They are nearly identical in dilute aqueous solution and diverge as concentration rises, because solute takes up volume.

Around 285 to 295 mOsm/L. Normal saline at 0.9% and 5% dextrose both land close to that range, which is why they can be infused without damaging cells.

Water moves into the cell, which swells and can burst. In a hypertonic solution water leaves and the cell shrinks. Matching osmolarity avoids both.

No. Osmotic effects are colligative, meaning they depend only on how many particles are present, not what they are. A mole of glucose and a mole of urea contribute equally.

By freezing-point depression. One osmole per kilogram of water lowers the freezing point by 1.86 °C, so an osmometer measures the depression and converts. Vapour pressure osmometry is an alternative and gives the same quantity.

The difference between measured osmolality and osmolarity calculated from sodium, glucose and urea. A gap above about 10 mOsm/kg suggests an unmeasured osmotically active substance is present — most often ethanol, methanol or ethylene glycol.

Because it is roughly isotonic with plasma, not because of any relation to normality as a concentration unit. At 0.154 M it delivers about 308 mOsm/L, close to plasma's 285 to 295.

It damages the vein wall, causing phlebitis. Solutions above roughly 900 mOsm/L are given through a central line, where rapid blood flow dilutes them quickly enough to avoid it.

Related but not the same. Osmotic pressure is the pressure needed to stop osmosis, given by π = iMRT, so it is proportional to osmolarity and depends on temperature as well. Plasma's roughly 290 mOsm/L corresponds to about 7.3 atm at body temperature.

Almost nothing — under 2 mOsm/L, because osmolarity counts particles and a protein is one large particle. That small contribution is oncotic pressure, and it matters enormously across the capillary wall precisely because proteins are the only solutes that cannot cross it.