Titrations

volume of solution (dm3)

Calculating concentrations

The concentration of a solution can be measured in grams per dm3 (g/dm3) or moles per dm3 (mol/dm3).

The following equation gives concentration in g/dm3:

If 1.0g of solid sodium hydroxide is dissolved in 250cm3 of

solution, what is the concentration in g/dm3?

concentration = 1/0.25 = 4g/dm3

concentration = mass dissolved (g)

mass of solid = 1g

Calculating concentrations in mol/dm

To calculate concentration in mol/dm3:





x

The equation for

concentration can be put into a formula

triangle:

volume of solution (dm3)

concentration = mass dissolved (mol)

Calculating concentration

If 1.0g of solid sodium hydroxide are dissolved in 250cm3 of

solution, what is the concentration in mol/dm3?

moles = mass/RAM = 1/40 = 0.025mol

 Calculate concentration:

concentration = moles/volume = 0.025/0.25 = 0.1mol/dm3

 Convert mass of solid into moles of solid:

RAM of sodium hydroxide = 40

Titration

Titration is a technique that can be used to accurately analyse the concentrations of substances in solution.

Titrations are often carried out by using a neutralization

reaction between an acid and an alkali.

How does titration work?

Imagine you have a sample of hydrochloric acid and you need to know its concentration. First, measure out a specific amount of the acid, and neutralize it with a standard

solution of an alkali. Measuring the amount of alkali that is needed to

neutralize the acid will allow you to work out the concentration of the acid.

If you measured out 25cm3 of the unknown acid, and found

that it was neutralized by 20cm3 of 0.1mol/dm3 alkali, is the

Attempt 3

Titration calculations

reading (cm3₎

volume of NaOH

added (cm3)

final burette

reading (cm3)

Titration Attempt 1 Attempt 2

0.0 19.9 19.9 19.9 20.1 40.0 How are the results of a titration used to calculate the concentration of an unknown acid solution?

Titration calculations

Result: 20cm3 of NaOH neutralizes 25cm3 of HCl of unknown

concentration.

1. Write a balanced equation for the reaction:

HCl + NaOH → NaCl + H

O

2. Calculate the number of moles of alkali:

moles = concentration (mol/dm-3) × volume (dm3)

= 0.1 × (20.00 / 1000) = 0.002 moles NaOH

Titration calculations

3. The balanced equation shows that one mole of HCl reacts with one mole of NaOH, so 0.002 moles of NaOH will

react with 0.002 moles of HCl.

4. Calculate the concentration of the HCl solution in mol/dm3:

= 0.002 / (25 / 1000) = 0.08 mol/dm3

concentration = moles

How would the pH curve look if you started with a strong alkali in the conical flask and added the strong acid to it from the burette?

pH curve for strong alkali and strong acid

0 5 10 15 20 25 30 35 40 45 50

0 7

pH

14

End points

The endpoint of a titration is not always at pH 7.

Titrations involving a weak acid or alkali can cause the indicator to change colour at a different pH.

strong strong weak strong weak strong

Acid Alkali pH at endpoint

7

more than 7 less than 7

weak

When choosing an indicator for a titration, it is important to match its pH range (the pH values where it changes colour) to the endpoint of the titration.

Why is litmus not a

particularly good indicator for a titration?

Indicators and pH ranges

 Phenolphthalein has a pH range of 8–9.

 Methyl orange has a pH range of 3–4.

 Litmus has a pH range of 5–8.