Moles and titrations

Moles and titration questions

A titration is a means by which an acid or base of known concentration and volume is reacted with a base or acid of unknown concentration. An indicator is used to tell us when the reaction is complete so that we know the volume that is required and the concentration can be calculated.

You have to be able to calculate concentrations and to be able to use moles effectively

Remember:   The number of moles  =  the mass / the mass of one mole

A molar  solution is a solution with one mole dissolved in a litre (mol/L). Usually we use mol dm^-3

E.g. a 2.0M solution means 2.0 moles per litre i.e. 2.0 moles in a 1000cm³  (2.0 mol dm^-3)

If you know the volume and the concentration you can work out the number of moles by:

Number of moles = concentration  x  volume in litres

E.g. 20.0 cm³ of a 0.25 mol dm^-3 solution has 0.25 x 0.020 = 5 x 10^-3 moles

By looking at the mole ratio in the equation you can see how many moles of the reactants and products you have, for example if there are 5.0 x10^-3   moles of sodium hydroxide how many moles of sulphuric acid would react with it?  To do this you need to write down a balanced equation

2NaOH  +  H₂SO4  →  Na₂SO₄ + 2H₂0

Therefore the number of moles of sulphuric acid that would react with 5.0 x 10^-3  moles of sodium hydroxide is 5.0 x 10^-3 / 2   = 2.5 x 10^-3

To work out concentrations you simply use:

concentration = number moles/ volume in litres

Remember 1 litre is 1,000 cm³

Therefore if there are 2.5 x10^-3 moles sulphuric acid in 40cm³ of solution then the concentration = 2.5 x10^-3  / 0.04 = 0.0625 mol dm^-3

Examples

1.   If 15.0 cm³  of 2M hydrochloric acid reacted exactly with 40.0 cm³ of potassium hydroxide. Calculate the concentration of the alkali

Remember there are four simple steps:

• Write a balanced equation
• Calculate the number of moles of the solution with a known concentration
• Determine the number of moles of the unknown solution by using the mole ratio
• Calculate the concentration of the unknown solution

                               c=n/v        n=cxv         v =n/c   

  

The volume must be in litres. If it’s in cm³   then divide by a 1,000

2.  If 25.0 cm³  of  0.50M sulphuric acid reacted with exactly 12.5 cm³  of  sodium hydroxide, calculate the concentration of the alkali.