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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
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