Molarity

Key Stage 3

Meaning

Molarity is aunit of concentration of solutions.

About Molarity

Molarity is shortened with an upper case M.

When molarity is used to describe a solution it may be referred to as molar, eg. a 0.5 molar solution of Sodium Chloride.

Bottles containing acid and alkali usually have their Molarity stated on the side. The more concentrated the higher the molarity.

The molarity of Stomach Acid is around 0.16 Molar (0.16M).

The molarity of Battery Acid can be as much as 5.2 Molar (5.2M).

Key Stage 4

Meaning

Molarity is a unit of concentration of solutions in mol/dm³.

About Molarity

Molarity is shortened with an upper case M.

Molarity is the ratio of the number of moles of a solute to the volume of solvent.

When molarity is used to describe a solution it may be referred to as molar, eg. a 0.5 molar solution of Sodium Chloride.

Bottles containing acid and alkali usually have their Molarity stated on the side. The more concentrated the higher the molarity.

Equation

concentration (mol/dm³) = \(\frac{Moles (mol)}{volume (dm^3)}\)

Where:

Moles = The number of moles of the solute. Found by dividing the mass in grams by the relative formula mass.

volume = The volume of solvent.

Calculating Molarity

117g of NaCl is dissolved in 0.5dm3 of water. Calculate the concentration in g/dm3.	28g of KOH is dissolved in 100ml of water. Calculate the concentration in g/dm3.	7.3g of HCl is dissolved in 400ml of water. Calculate the concentration in g/dm3.
Find the number of moles. mass = 117g Relative Formula Mass = 58.5g Number of Moles of a Compound = (Mass of compound)/(Relative Formula Mass of compound) Number of Moles = \({\frac{m}{M_r}}\) Number of Moles = \({\frac{117}{58.5}}\) Number of Moles = 2mol	State the mass in grams and the volume in dm3: Find the number of moles. mass = 28g Relative Formula Mass = 56g Number of Moles of a Compound = (Mass of compound)/(Relative Formula Mass of compound) Number of Moles = \({\frac{m}{M_r}}\) Number of Moles = \({\frac{28}{56}}\) Number of Moles = 0.5mol	Find the number of moles. mass = 7.3g Relative Formula Mass = 36.5g Number of Moles of a Compound = (Mass of compound)/(Relative Formula Mass of compound) Number of Moles = \({\frac{m}{M_r}}\) Number of Moles = \({\frac{7.3}{36.5}}\) Number of Moles = 0.2mol
volume = 0.5dm3	volume = 100ml The volume must be converted into dm3 from ml. volume in dm3 = (volume in ml)/1000 volume = 0.1dm3	volume = 400ml The volume must be converted into dm3 from ml. volume in dm3 = (volume in ml)/1000 volume = 0.4dm3
Calculate the Concentration: concentration (mol/dm3) = \(\frac{Moles (mol)}{volume (dm^3)}\) concentration = \(\frac{2}{0.5}\) concentration = 4mol/dm3 = 4M	Calculate the Concentration: concentration (mol/dm3) = \(\frac{Moles (mol)}{volume (dm^3)}\) concentration = \(\frac{0.5}{0.1}\) concentration = 5mol/dm3 = 5M	Calculate the Concentration: concentration (mol/dm3) = \(\frac{mass (g)}{volume (dm^3)}\) concentration = \(\frac{0.2}{0.4}\) concentration = 0.5mol/dm3 = 5M