Molarity calculator
This molarity calculator finds the amount concentration in mol/L. 3.646 g of hydrogen chloride (M = 36.46 g/mol) in 100 mL of solution gives 1 mol/L. It also calculates how much to weigh out for a wanted solution, molarity from mass percent and density, and simple stoichiometry.
Example: What do you want to calculate: Molarity from mass and volume · Molar mass M (solute or substance A) 36.46 g/mol · Mass of the solute 3.646 g · Volume of the solution 100 mL → Result: c = 1 mol/L. Source: IUPAC Gold Book – amount concentration (c = n/V) and molar mass. Updated: .
How it is calculated
Calculating molarity: the formulas
Molarity (amount concentration) is c = n ÷ V in mol/L. The amount follows from the mass: n = m ÷ M. Together: c = m ÷ (M × V). To make V liters of solution you weigh out m = c × V × M.
- 5.844 g NaCl (M = 58.44 g/mol) in 100 mL: n = 0.1 mol, c = 1 mol/L
- 0.5 mol/L HCl, 100 mL: weigh out 1.823 g
- 0.1 mol/L NaOH, 500 mL: weigh out 2.000 g
Molarity from mass percent and density
Commercial acids are labelled in mass percent. From the mass fraction w and the density ρ (g/mL): c = w × ρ × 10 ÷ M. Example: hydrochloric acid 37% (ρ = 1.19 g/mL) is about 12.08 mol/L, sulfuric acid 98% (ρ = 1.84 g/mL) about 18.4 mol/L. To make 1 L of 1 mol/L hydrochloric acid you therefore need roughly 83 mL of the concentrated acid.
Stoichiometry
The reaction equation gives the mole ratio: n(B) = n(A) × ν(B) ÷ ν(A). From it you can calculate the mass of a reaction partner, for example 3.646 g HCl with NaOH giving 5.844 g NaCl.
Molarity, molality and normality
Molarity refers to the volume of the solution and therefore changes with temperature. Molality (mol per kg of solvent) does not. Normality is considered obsolete; today mol/L is used.
Honest limits
The calculator takes molar mass and density as you enter them; for hydrates (such as CuSO₄·5H₂O) the water of crystallisation must be included in the molar mass. It does not replace a laboratory procedure.
Frequently asked questions
How do you calculate molarity?
c = n ÷ V, with n = m ÷ M. 5.844 g NaCl in 100 mL of solution is 0.1 mol in 0.1 L, so 1 mol/L.
How much do I weigh out for a solution?
m = c × V × M. For 500 mL of 0.1 mol/L NaOH: 0.1 × 0.5 × 40.00 = 2.00 g.
How do I calculate molarity from density?
c = mass percent × density (g/mL) × 10 ÷ molar mass. Hydrochloric acid 37% at 1.19 g/mL gives 12.08 mol/L.
What is the difference between molarity and molality?
Molarity is mol per liter of solution, molality mol per kilogram of solvent. Only molality is independent of temperature.
Where do I find the molar mass?
It comes from the formula and the atomic masses. The “Molar mass” tool calculates it from the formula; then enter the number here.
Sources and legal basis
- IUPAC Gold Book – amount concentration (c = n/V) and molar mass
- BIPM – SI Brochure: mole, amount of substance
- IUPAC CIAAW – Standard Atomic Weights (atomic masses for the molar mass)
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