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bondistry

Molarity calculator

Find molarity from mass, molar mass and volume, or work out how many grams to weigh for the concentration you need. Type a formula and the molar mass fills in by itself.

Type a formula and the molar mass fills in automatically.

g/mol

from NaCl

↑ calculated

RESULT · Molarity

0.2M

Step-by-step solution

  1. 01
    Get the molar mass from the formula: M(NaCl) = 58.44 g/mol see steps →
  2. 02
    Convert the values to g, L and M:V = 500 mL = 0.5 L
  3. 03
    Molarity is moles per litre: C = n / V = m / (M × V)
  4. 04
    Substitute the values: C = 5.844 g / (58.44 g/mol × 0.5 L) = 0.2 MAmount of substance: n = m / M = 0.1 mol

What is molarity?

Molarity (molar concentration) is the number of moles of solute per litre of solution. Its unit is mol/L, written simply as M: every litre of a 1 M solution contains 1 mole of solute. It is the most widely used concentration unit in chemistry because reactions depend on numbers of particles, and molarity ties that number directly to volume.

The denominator in molarity is the total volume of solution, not of solvent. To make a 1 M solution you dissolve 1 mole of substance and then make the total volume up to 1 litre, rather than adding it to 1 litre of water.

How to calculate molarity

The basic relation is C = n / V, where n is the number of moles and V the volume in litres. Moles come from mass divided by molar mass: n = m / M. Combining them gives C = m / (M × V). Knowing any three of these four values lets you find the fourth.

The reverse question is just as common: how many grams do you weigh to make a solution of a given molarity? Rearranged for mass, the equation becomes m = C × V × M. In the calculator, enter the concentration, volume and formula and leave the mass field empty.

When you type a formula such as NaCl or CuSO4·5H2O, the molar mass is calculated automatically from the same PubChem data as the molar mass calculator. If you are weighing a hydrated salt, include its water in the formula, otherwise the concentration will be wrong.

Units

Choose mass in g or mg, volume in L, mL or µL, and concentration in M, mM or µM. The calculation converts everything to g, L and M first, shows the result in your chosen unit and writes the conversion into the step-by-step solution.

Worked examples

Example 1

Molarity from a weighed salt

5.844 g of NaCl is dissolved in water and made up to 500 mL. What is the molarity?

  1. 01Molar mass: M(NaCl) = 58.44 g/mol.
  2. 02Moles: n = 5.844 g / 58.44 g/mol = 0.1000 mol.
  3. 03Volume: 500 mL = 0.500 L.
  4. 04Molarity: C = 0.1000 mol / 0.500 L = 0.2000 M.

Answer:C = 0.2 M

Example 2

Mass needed for a target molarity

How many grams of NaOH do you weigh to make 250 mL of 0.1 M NaOH?

  1. 01Molar mass: M(NaOH) = 39.997 g/mol.
  2. 02Moles needed: n = C × V = 0.1 M × 0.250 L = 0.0250 mol.
  3. 03Mass: m = 0.0250 mol × 39.997 g/mol = 0.9999 g.

Answer:m ≈ 1.000 g