Molarity Calculator

How many grams to weigh for a molar solution, or the molarity of what you made.

Mass (g) = molarity (mol/L) × volume (L) × molecular weight (g/mol). For 500 mL of 0.5 M NaCl (58.44 g/mol): 0.5 × 0.5 × 58.44 = 14.61 g. Rearranged, molarity = (mass ÷ molecular weight) ÷ volume.

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How to use it

Choose what to solve for: mass, molarity or volume.
Enter the molecular (formula) weight in g/mol and the other two values with their units.
Weigh, dissolve in less than the final volume, then make up to the mark.

What you enter

Molecular weight (g/mol)
Two of: mass, concentration, final volume

What you get

The third value with sensible units
The working as an equation

Formula

mass (g) = M (mol/L) × V (L) × MW (g/mol)  |  M = mass ÷ MW ÷ V  |  V = mass ÷ MW ÷ M

Convert every value to grams, litres and mol/L first; the calculator does this for you.

Worked example

Input: 0.5 M NaCl, 500 mL, MW 58.44 g/mol

Result: Weigh 14.61 g

Dissolve in about 400 mL water, then make up to 500 mL.

Questions

How do you calculate molarity?

Divide the moles of solute by the volume of solution in litres. Moles are mass in grams divided by molecular weight. Example: 14.61 g NaCl ÷ 58.44 g/mol = 0.25 mol; in 0.5 L that is 0.5 M.

How many grams do I need for a 1 M solution?

The molecular weight in grams, per litre. A 1 M NaCl solution needs 58.44 g per litre; 100 mL needs 5.844 g.

Should I use the hydrated or anhydrous molecular weight?

Use the formula weight on the bottle you are weighing from. CaCl₂·2H₂O is 147.01 g/mol, anhydrous CaCl₂ is 110.98 g/mol; using the wrong one puts the concentration 25–30% off.

Limitations

Assumes the solute dissolves completely and its purity is 100%.
For dilutions from a stock, use a C1V1 = C2V2 dilution calculator.

References

Green MR, Sambrook J (2012) Cold Spring Harbor Laboratory Press, 4th ed.

More Chemistry

All free tools · By Dr. Zubair Khalid, DVM, MS, PhD.