OneTaskBy Tim King · Updated
Molarity Calculator
Grams to Weigh for a Target Molarity and the Volume You Need
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How the Molarity Calculator works
This molarity calculator works out the concentration of a solution in moles per litre from the mass of solute and the volume, or tells you how many grams to weigh out to make a solution of a given molarity. Type the solute's formula and it finds the molar mass for you, then shows the working, the millimolar value and the mass concentration in g/L.
Molarity, written M, is the standard way to describe how concentrated a solution is in a chemistry lab. A 1 M solution has one mole of solute in every litre of finished solution, so it tells you directly how many particles you are adding when you measure out a volume.
How the molarity calculator works
The formula is molarity = moles of solute ÷ litres of solution, and moles = grams ÷ molar mass. Turning it round, the mass to weigh is molarity × litres × molar mass, and the volume is moles ÷ molarity. Volumes are entered in millilitres and converted to litres for you.
First example. 5.844 g of NaCl is 5.844 ÷ 58.44 = 0.1 mol. Made up to 250 mL (0.25 L), that is 0.1 ÷ 0.25 = 0.4 M, or 23.4 g/L.
Second example. To make 500 mL of 0.2 M copper sulfate from the pentahydrate, CuSO4·5H2O at 249.677 g/mol, weigh 0.2 × 0.5 × 249.677 = 24.97 g and make up to the mark.
How many grams of NaOH make 1 L of 1 M solution? NaOH is 39.997 g/mol, so you need 40.0 g.
Grams of solute to weigh for common lab solutions
| Solute | Formula | Molar mass (g/mol) | Grams for 1 L of 1 M | Grams for 250 mL of 0.1 M | Grams for 100 mL of 0.5 M |
|---|---|---|---|---|---|
| Sodium chloride | NaCl | 58.44 | 58.44 | 1.461 | 2.922 |
| Potassium chloride | KCl | 74.55 | 74.55 | 1.864 | 3.727 |
| Sodium hydroxide | NaOH | 40.00 | 40.00 | 1.000 | 2.000 |
| Potassium hydroxide | KOH | 56.11 | 56.11 | 1.403 | 2.805 |
| Glucose | C6H12O6 | 180.16 | 180.16 | 4.504 | 9.008 |
| Sucrose | C12H22O11 | 342.30 | 342.30 | 8.557 | 17.115 |
| Sodium carbonate | Na2CO3 | 105.99 | 105.99 | 2.650 | 5.299 |
| Sodium bicarbonate | NaHCO3 | 84.01 | 84.01 | 2.100 | 4.200 |
| Calcium chloride | CaCl2 | 110.98 | 110.98 | 2.774 | 5.549 |
| Magnesium sulfate | MgSO4 | 120.36 | 120.36 | 3.009 | 6.018 |
| Copper(II) sulfate pentahydrate | CuSO4·5H2O | 249.68 | 249.68 | 6.242 | 12.484 |
| Potassium nitrate | KNO3 | 101.10 | 101.10 | 2.528 | 5.055 |
| Silver nitrate | AgNO3 | 169.87 | 169.87 | 4.247 | 8.494 |
| Ammonium chloride | NH4Cl | 53.49 | 53.49 | 1.337 | 2.674 |
| Potassium permanganate | KMnO4 | 158.03 | 158.03 | 3.951 | 7.902 |
| Sodium acetate | CH3COONa | 82.03 | 82.03 | 2.051 | 4.102 |
| Tris base | C4H11NO3 | 121.14 | 121.14 | 3.028 | 6.057 |
| EDTA disodium dihydrate | C10H14N2Na2O8·2H2O | 372.24 | 372.24 | 9.306 | 18.612 |
| Urea | CH4N2O | 60.06 | 60.06 | 1.501 | 3.003 |
| Potassium dihydrogen phosphate | KH2PO4 | 136.08 | 136.08 | 3.402 | 6.804 |
| Sodium thiosulfate pentahydrate | Na2S2O3·5H2O | 248.17 | 248.17 | 6.204 | 12.409 |
| Iron(II) sulfate heptahydrate | FeSO4·7H2O | 278.01 | 278.01 | 6.950 | 13.900 |
Tips and pitfalls
- Volume of solution, not solvent. Dissolve the solid, then add water up to the final volume in a volumetric flask. Adding 250 mL of water to the solid gives a slightly different volume.
- Use the right hydrate. Check the bottle label; the formula on it sets the molar mass.
- Millilitres to litres. Divide by 1,000. Forgetting this is the most common molarity mistake.
- Molarity changes with temperature. Solutions expand when warm, so precise work uses molality (per kg of solvent) instead.
FAQ
How do you calculate molarity?
Divide the moles of solute by the volume of solution in litres. If you have a mass instead of moles, divide the mass by the molar mass first. For 10 g of glucose, C6H12O6 at 180.16 g/mol, in 500 mL: 10 ÷ 180.16 = 0.0555 mol, then 0.0555 ÷ 0.5 = 0.111 M.
What does 1 M mean?
1 M, read as one molar, means one mole of solute per litre of solution. For sodium chloride that is 58.44 g dissolved and made up to exactly one litre. A 0.1 M solution has a tenth as much solute in the same volume.
How do I make a solution of a given molarity?
Multiply the molarity you want by the volume in litres and by the molar mass to get the grams to weigh. Dissolve that mass in less water than you need, transfer to a volumetric flask and top up to the line, then mix well.
What is the difference between molarity and molality?
Molarity is moles per litre of solution, while molality is moles per kilogram of solvent. Molality does not change with temperature because masses do not expand, so it is used for freezing point and boiling point calculations.
How do you convert molarity to g/L?
Multiply the molarity by the molar mass. A 0.5 M solution of potassium chloride, KCl at 74.55 g/mol, contains 0.5 × 74.55 = 37.3 g per litre. The calculator shows this mass concentration and the percent weight per volume with every answer.
See also: all Chemistry tools, Physics, Electronics.
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