Molarity Calculator
Enter solute mass, molar mass and solution volume to instantly compute moles of solute and molarity (mol/L, i.e. M), with a reference table of common reagent molar masses.
Input Data
Results
At a glance:Molarity (symbol M or c) is the number of moles of solute per litre of solution. It is the most common concentration unit in chemistry and molecular-biology experiments. Calculation takes two steps: first find moles from mass and molar mass, n = m / MW, then divide by solution volume to get concentration c = n / V. The unit is mol/L (M); for example 1 M NaCl contains 1 mol (about 58.44 g) of NaCl per litre of solution.
Formula
Moles of solute: n = m ÷ MW (m in g, MW in g/mol).
Molarity: c = n ÷ V (V in L).
Combined: c = m ÷ (MW × V).
$$c = \dfrac{n}{V} = \dfrac{m}{MW \times V}$$$$n = \dfrac{m}{MW}$$How to Use
- Enter the weighed solute mass m (g).
- Enter the solute molar mass MW (from the periodic table or reagent bottle) and the prepared solution volume V (L).
- The right panel instantly shows moles of solute and molarity (mol/L).
Molar mass reference for common lab reagents
| Reagent | Formula | Molar Mass (g/mol) |
|---|---|---|
| Sodium chloride (table salt) | NaCl | 58.44 |
| Glucose | C₆H₁₂O₆ | 180.16 |
| Potassium chloride | KCl | 74.55 |
| Hydrochloric acid | HCl | 36.46 |
| Sodium hydroxide | NaOH | 40.00 |
| Tris base | C₄H₁₁NO₃ | 121.14 |
Molar mass is the sum of atomic weights from the periodic table, or read directly from the reagent bottle (Formula Weight, FW).
Case Studies
Preparing 1 M NaCl solution
To make 1 L of 1 M sodium chloride, NaCl molar mass is 58.44 g/mol.
Required mass m = c × MW × V = 1 × 58.44 × 1 = 58.44 g; weigh 58.44 g NaCl, dissolve in water and make up to 1 L.
Check: n = 58.44 ÷ 58.44 = 1 mol, c = 1 ÷ 1 = 1 mol/L, matches the target concentration.
Concentration of a dilute solution
Dissolve 5 g hydrochloric acid (HCl, MW=36.46) in water and make up to 1.2 L.
Moles n = 5 ÷ 36.46 ≈ 0.1371 mol, concentration c = 0.1371 ÷ 1.2 ≈ 0.1143 mol/L.
This is about 0.11 M, suitable as a dilute acid stock for titration or reactions, diluted further as needed.
FAQ
How does molarity differ from mass concentration?
Molarity expresses the number of moles per litre of solution (mol/L), reflecting the number of molecules; mass concentration expresses mass per litre (e.g. g/L or mg/mL). The two convert via molar mass: mass concentration = molarity × molar mass. Chemical reactions proceed by mole ratios, so experiments mostly use molarity.
Should volume be the total solution volume or the solvent volume?
Use the total volume of the prepared solution, not the volume of added water (solvent). The correct method is to dissolve the solute first, then add water to the target mark, because the dissolved solute also occupies volume. This is also the difference between molarity and molality (which uses solvent mass).
How do I look up molar mass (MW)?
Sum the atomic weights of each atom in the formula (periodic table), e.g. NaCl = 22.99 + 35.45 = 58.44 g/mol; or read the Formula Weight (FW) on the reagent bottle. This calculator requires you to enter the molar mass yourself.
How much is 1 M vs 1 mM?
1 M (mol/L) = 1000 mM (mmol/L) = 1,000,000 μM. Molecular-biology concentrations are usually in the mM to μM range; remember 1 M = 10³ mM = 10⁶ μM so you do not misprepare by a 1000× factor.
Can this be used for percentage concentration (%)?
This calculator computes molarity. Percentage concentration (e.g. 0.9% saline = 0.9 g per 100 mL) is a mass/volume concentration that must be converted first: 0.9% NaCl = 9 g/L, divided by 58.44 g/mol ≈ 0.154 M. Convert % to g/L before entering.
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References
Content review: Calculatorism Science Team. Results are for reference only; please refer to the relevant authorities for the official figures.