Molar Concentrations – The Hard Part

Remember that:

Mol/dm³ means moles per litre (e.g. a salt solution of 0.5 mol/dm³ means 0.5 moles (or 58.5/2 = 29.25g) of salt was dissolved in a litre of water
It’s all about proportion

20 cm³ of 0.5 mol dm³sodium hydroxide solution was dissolved with 25cm³ of hydrochloric acid to form a sodium chloride solution. Calculate the concentration of HCl needed to react with the NaOH

NaOH + HCl -> H₂O + NaCl

RFM of NaOH = 40g = 1 mole of NaOH

0.5 mol dm³ of NaOH means (40 x 0.5) 20g of NaOH was dissolved in 1000cm³ of water

The amount of moles in 20cm³ of NaOH solution:

20cm³/1000cm³ x 0.5 moles = 1/50 x 0.5 = 0.01 moles of NaOH

The equation says that 1 mole of NaOH + 1 mole of HCl gives 1 mole of NaCl

So 0.01 moles of NaOH + 0.01 moles of HCl gives 0.01 moles of NaCl

So 0.01 moles of HCl was present in 25cm³ of HCl solution! However, concentration is measured in mol dm³ so:

1000cm³/25cm³ x 0.01 mol = 0.4 mol dm³ of HCl used.