Molarity Calculator

The formula

Moles = mass (g) ÷ molar mass (g/mol). Molarity = moles ÷ volume (L).

Worked example

5.85 g of table salt (NaCl, molar mass 58.5 g/mol) dissolved in 0.5 L of water gives 5.85 ÷ 58.5 = 0.1 mol of NaCl, and a molarity of 0.1 ÷ 0.5 = 0.2 mol/L.

Inputs, units and rounding

• Mass is in grams, molar mass in g/mol and volume in liters. There is no unit selector, so 250 mL must be typed as 0.25.
• All three boxes are required and each must be greater than zero; otherwise you get a message asking for positive values.
• Moles and molarity are both shown to four decimal places, and molarity is labelled mol/L.
• The molar mass is not looked up for you. Add up the atomic masses for your formula and type the total.

Three worked examples

40 g of sodium hydroxide (molar mass 40 g/mol) made up to 2 L is 40 ÷ 40 = 1.0000 mol, so the molarity is 1 ÷ 2 = 0.5000 mol/L.

9.8 g of sulfuric acid (98.08 g/mol) in 0.25 L gives 0.0999 mol and 0.3997 mol/L. The moles come out just under 0.1 because 9.8 is a little less than one tenth of 98.08.

The unit slip to avoid: 250 g of glucose (180.16 g/mol) in 500 mL. Typing 0.5 for the volume gives 1.3877 mol and 2.7753 mol/L. Typing 500 by mistake gives 0.0028 mol/L, which is a thousand times too small.

Limits

This is molarity (moles per liter of final solution), not molality, which divides by the mass of solvent. The tool also does not adjust for purity, water of crystallization or temperature, so use the molar mass of the exact form you weighed out.

Examples

5.844 g of sodium chloride, with a molar mass of 58.44 g/mol, is 0.1000 mol, and dissolved to make 1 liter of solution that is 0.1000 mol/L. 10 g of a compound with a molar mass of 40 g/mol in 0.5 liter gives 0.2500 mol and 0.5000 mol/L.

The volume must be the final volume of the solution in liters, not the volume of solvent you started with, and a volume of zero returns "Please enter positive values." Results are shown to four decimal places.