Molarity Calculator

Concentration and Molarity

When chemists mix solutions, they rarely care about the absolute volume of the liquid as much as they care about the concentration—how densely packed the reactive molecules are within that liquid.

Molarity (denoted by a capital $M$) is the most universally standard unit of concentration in chemistry. It measures exactly how many moles of a dissolved substance (the solute) are present in every single liter of the total liquid mixture (the solution).

A "1 Molar" ($1 \text{M}$) solution contains exactly 1 mole of solute per liter of solution. A "0.5 Molar" ($0.5 \text{M}$) solution contains exactly half a mole of solute per liter.

Why Molarity is Essential

Molarity is crucial for stoichiometry because it allows chemists to measure out precise numbers of molecules using a simple liquid measuring cylinder, rather than having to dry out a powder and weigh it on a scale. If you know you need $0.1 \, \text{moles}$ of Hydrochloric Acid for a reaction, and you have a $2 \text{M}$ stock solution, you know instantly that you need exactly $50 \, \text{mL}$ of the liquid.

How to Calculate Molarity

To calculate Molarity, you need two pieces of information:

1. The number of moles of your solute. (If you only know the mass in grams, you must first divide it by the solute's Molar Mass).
2. The total volume of the resulting solution, measured strictly in Liters.

The Formula

Example Calculation

Let's say you dissolve $58.44 \, \text{grams}$ of table salt (NaCl) into enough water to make exactly $2.0 \, \text{Liters}$ of total solution. What is the molarity?

1. Find the Molar Mass of NaCl: (Na = 22.99, Cl = 35.45) = $58.44 \, \text{g/mol}$.
2. Convert grams to moles: $58.44 \, \text{g} / 58.44 \, \text{g/mol} = 1.0 \, \text{moles of NaCl}$.
3. Divide by Volume: $1.0 \, \text{moles} / 2.0 \, \text{Liters} = \mathbf{0.5 \, \text{M}}$.

The resulting salt water is a $0.5 \text{M}$ solution.