Molarity Calculator

How to use this molarity calculator

1. Choose what to solve for: molarity, mass of solute, or volume of solution.
2. Pick a compound to fill in its molar mass, or type the molar mass yourself.
3. Enter the other two values. Choose mL or L for the volume.

The answer, the moles of solute and every step of the math update as you type.

Molarity formula

Molarity (M) is the number of moles of solute in one liter of solution. Its unit is mol/L, written M.

Molarity = moles of solute ÷ liters of solution, and moles = mass ÷ molar mass

Combine the two and you can solve for any one value from the other three:

• Molarity: M = mass ÷ (molar mass × volume in L)
• Mass of solute: mass = M × volume in L × molar mass
• Volume of solution: volume in L = mass ÷ (molar mass × M)

How to calculate molarity: worked examples

Find the molarity. You dissolve 5.844 g of sodium chloride (NaCl, 58.44 g/mol) in water to make 250 mL of solution.

• Moles: 5.844 g ÷ 58.44 g/mol = 0.1000 mol
• Volume: 250 mL = 0.250 L
• Molarity: 0.1000 mol ÷ 0.250 L = 0.400 M

Find the mass. You need 500 mL of 0.100 M sodium hydroxide (NaOH, 40.00 g/mol).

• Moles: 0.100 M × 0.500 L = 0.0500 mol
• Mass: 0.0500 mol × 40.00 g/mol = 2.00 g

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Molar masses of common compounds

Molar masses are calculated from IUPAC standard atomic weights. To find a molar mass yourself, add the atomic masses of every atom in the formula. For NaCl: 22.99 + 35.45 = 58.44 g/mol.

Dilution: M1V1 = M2V2

When you dilute a solution, the moles of solute stay the same. Only the volume changes, so M1 × V1 = M2 × V2.

Example: you dilute 10.0 mL of 2.00 M HCl to a total of 100.0 mL. The new molarity is 2.00 M × 10.0 mL ÷ 100.0 mL = 0.200 M.

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Common molarity mistakes

• Forgetting to convert mL to L. 250 mL is 0.250 L. Using 250 gives an answer 1,000 times too small.
• Using the volume of water, not of solution. Molarity uses the final volume of the whole solution. To make 1.00 L of 1.00 M NaCl, dissolve 58.44 g of NaCl and add water until the total volume is 1.00 L.
• Using grams instead of moles. Divide the mass by the molar mass first.
• Mixing up molarity and molality. Molarity is moles per liter of solution. Molality is moles per kilogram of solvent.

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