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Chemistry · Ch 1 — Some Basic Concepts of Chemistry

Molarity

3

Molarity

Molarity (M)

Molarity is the most widely used concentration unit in the laboratory, denoted by M. It is defined as the number of moles of solute per litre of solution:

M=Number of moles of soluteVolume of solution in litresM = \frac{\text{Number of moles of solute}}{\text{Volume of solution in litres}}

Watch out

Molarity depends on temperature, because the volume of a solution is temperature-dependent. A solution that is 1 M at 25 °C will not be exactly 1 M at 50 °C.

Preparing a Dilute Solution from a Stock Solution

Suppose we have a 1 M solution of NaOH and want to prepare a 0.2 M solution from it. 1 M NaOH means 1 mol of NaOH is present in 1 litre of solution; for a 0.2 M solution we require 0.2 mol of NaOH dissolved in 1 litre of solution. So we take that volume of the 1 M stock which contains 0.2 mol of NaOH and dilute it with water to 1 litre:

If 1 mol is present in 1 L (1000 mL) of solution, then 0.2 mol is present in

1000 mL1 mol×0.2 mol=200 mL\frac{1000\ \text{mL}}{1\ \text{mol}} \times 0.2\ \text{mol} = 200\ \text{mL}

Thus, 200 mL of 1 M NaOH is taken, and enough water is added to dilute it to one litre.

In fact, for such calculations a general formula can be used:

M1V1=M2V2M_1 V_1 = M_2 V_2 …

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