Molarity
Contents
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/dm^{3}.
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/dm^{3}) = \(\frac{Moles (mol)}{volume (dm^3)}\)
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 0.5dm^{3} of water. Calculate the concentration in g/dm^{3}.  28g of KOH is dissolved in 100ml of water. Calculate the concentration in g/dm^{3}.  7.3g of HCl is dissolved in 400ml of water. Calculate the concentration in g/dm^{3}. 
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 dm^{3}: 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 = 0.5dm^{3} 
volume = 100ml The volume must be converted into dm^{3} from ml. volume in dm^{3} = (volume in ml)/1000 volume = 0.1dm^{3} 
volume = 400ml The volume must be converted into dm^{3} from ml. volume in dm^{3} = (volume in ml)/1000 volume = 0.4dm^{3} 
Calculate the Concentration: concentration (mol/dm^{3}) = \(\frac{Moles (mol)}{volume (dm^3)}\) concentration = \(\frac{2}{0.5}\) concentration = 4mol/dm^{3} = 4M 
Calculate the Concentration: concentration (mol/dm^{3}) = \(\frac{Moles (mol)}{volume (dm^3)}\) concentration = \(\frac{0.5}{0.1}\) concentration = 5mol/dm^{3} = 5M 
Calculate the Concentration: concentration (mol/dm^{3}) = \(\frac{mass (g)}{volume (dm^3)}\) concentration = \(\frac{0.2}{0.4}\) concentration = 0.5mol/dm^{3} = 5M 