## What is Molarity?

The molarity of a solution can be defined as the concentration of the solution in terms of the number of moles of the solute in 1 dm3 (or 1 liter) of the solution.

Mathematically, the molarity of a solution can be expressed as:

Molarity (M) = number of moles/volume (dm3 or liter) A 1 dm3 of solution which has 1 mole of solute dissolved in it is said to be 1 molar.

As can be deduced, molarity is another way of expressing the concentration of solutions of chemical compounds. The symbol of molarity is capital letter M.

### Calculating the Molarity of a Chemical Solution

Question 1: How many grams of sugar, C12H22O11, are needed to prepare 500 cm3 of 0.25 M solution?

Solution:

Molarity of solution = number of moles of solute/volume (dm3)

Number of moles of solute = molarity x volume (dm3)

Number of moles of sugar = 0.25 M x 0.5 dm3

= 0.125 moles

Note that the volume has to be changed from cm3 to dm3

From the equation;

Number of moles = conc. (g) / molar mass

Conc. (g) = number of moles x molar mass

Molar mass of sugar = 12 x 12 + 1 x 22 + 16 x 11 = 144 + 22 + 176 = 342

Molar mass of sugar = 342 g/mol

Click here for Tutorial on molar mass and how to calculate it.

Therefore; Restating:

Conc. (g) = number of moles x molar mass

Conc. (g) = 0.125 x 342 = 42.75 g

Question 2. What is the molarity of a solution prepared by dissolving 13.2 g of NaCl in sufficient water to give 2.00 liters?

Solution: First, determine the number of moles of NaCl in 13.2 g.

Number of moles = conc. (g) /molar mass

= 13.2/58.5 = 0.23 moles

Note that the molar mass of NaCl is 23 + 35.5 = 58.5 g/mol

Therefore,

Molarity = number of moles/volume (liter) = 0.23/2 = 0.115 M

Question 3. What volume of 2.00 M NaOH would be needed to furnish 12.0 g of NaOH for a reaction?

Solution: Convert 12.0 g of NaOH to number of moles using the equation:

Number of moles = conc. (g)/molar mass

Molar mass of NaOH = 23 + 16 + 1 = 40 g/mol

Therefore, Number of moles of NaOH = 12/40 = 0.30

From the equation,

Molarity = number of moles/volume (dm3)

Volume = number of moles/molarity

Volume = 0.3/2.0 = 0.15 dm3