Titrations (chemistry only)

What you'll learn:

What a titration is and the specialist glassware needed to perform one.
How to carry out a titration accurately (Required Practical 2).
How to select and use concordant results to find a mean titre.
(Higher Tier only) How to calculate unknown concentrations in moles per cubic decimetre and grams per cubic decimetre.

What is a titration?

You already know that when an acid reacts with an alkali, they undergo a neutralisation reaction to produce a salt and water. But what if you want to know exactly how much acid is needed to completely neutralise a specific amount of alkali?

Definition

Titration

A titration is an experimental technique used to find the exact volume of an acid that neutralises a known volume of an alkali (or vice versa).

By finding these exact reacting volumes, you can figure out the unknown concentration of either the acid or the alkali.

The specialist apparatus

To get highly accurate results, everyday measuring cylinders are not precise enough. We use two specific pieces of glassware: the pipette and the burette.

Pipette: A glass tube with a bulge in the middle, used to measure a single, fixed volume of liquid (usually 25.0 cm³) very accurately. You use a pipette filler to draw the liquid up safely.
Burette: A tall, thin glass tube with a tap at the bottom and markings all the way down. It is used to measure variable volumes drop by drop.

A diagram showing the setup for a titration

A conical flask is used instead of a beaker because its sloped sides allow you to swirl the liquid continuously without it splashing out. The white tile sits underneath the flask to make the colour change of the indicator easier to see.

Common Mistake

Using Universal Indicator

You cannot use Universal Indicator in a titration. Universal Indicator gives a gradual colour change (e.g., from blue to green to yellow). In a titration, you need a sudden, sharp colour change to know the exact drop that caused neutralisation. You must use a single indicator like phenolphthalein or methyl orange.

The method (Required Practical 2)

If you are asked to describe how to perform a titration, you need to lay out clear, logical steps. Imagine we are finding out how much hydrochloric acid is needed to neutralise 25.0 cm³ of sodium hydroxide.

Use a pipette and pipette filler to add 25.0 cm³ of sodium hydroxide (the alkali) into a clean conical flask.
Add a few drops of a suitable indicator (like phenolphthalein) to the flask and place it on a white tile.
Fill a burette with the hydrochloric acid. Make sure the tap is closed. Remove the funnel from the top of the burette so drops don't fall in later.
Read the initial volume on the burette. Always read from the bottom of the meniscus (the curve of the liquid).
Open the tap to slowly add the acid to the alkali, swirling the flask continuously.
As you get close to the end point, add the acid drop by drop.
Stop adding acid the moment the indicator permanently changes colour (phenolphthalein turns from pink to colourless).
Record the final volume on the burette.
Subtract the initial reading from the final reading to find the titre (the volume of acid added).

Tip

The rough titration

Your first attempt is usually a "rough titration" to get a rough idea of where the colour change happens. You don't use the rough titre in your final calculations; it just saves you time on your next, more accurate attempts.

Finding the mean titre

You must repeat the titration until you get concordant results.

Definition

Concordant results

Results that are extremely close together, specifically within 0.10 cm³ of each other.

Once you have at least two concordant results, you ignore the rough titration and any anomalies, and calculate a mean using only those concordant values.

Example

Calculating the mean titre

A student records the following titration results for the volume of acid added: Rough: 23.40 cm³ Run 1: 22.80 cm³ Run 2: 23.10 cm³ Run 3: 22.75 cm³ Run 4: 22.85 cm³ Calculate the mean titre.

Identify the concordant results. We need values that are within 0.10 cm³ of each other. Looking at the data, Run 1 (22.80 cm³), Run 3 (22.75 cm³), and Run 4 (22.85 cm³) are all within 0.10 cm³ of one another.
Discard the non-concordant results. We ignore the Rough run (23.40 cm³) and Run 2 (23.10 cm³).
Calculate the mean of the concordant results by adding them together and dividing by how many there are:

\begin{aligned} \text{Mean} &= \frac{22.80 + 22.75 + 22.85}{3} \\ &= \frac{68.40}{3} \\ &= 22.80 \text{ cm}^3 \end{aligned}

Titration calculations (Higher Tier only)

If you are taking the Higher Tier paper, you will need to use your titration results to calculate the concentration of the unknown solution.

You need to know the formula linking moles, concentration, and volume. Because concentrations are measured in moles per cubic decimetre ( $dm−3\text{mol dm}^{-3}$ ), your volumes must be in cubic decimetres ( $dm3\text{dm}^3$ ).

Key Idea

Converting volume units

In chemistry, volumes are usually measured in $cm3\text{cm}^3$ , but formulas need them in $dm3\text{dm}^3$ . To convert from $cm3\text{cm}^3$ to $dm3\text{dm}^3$ , divide by 1000.

Example

Calculating unknown concentration

In a titration, 25.0 cm³ of a $\text{ mol dm}^{-3}$ sodium hydroxide ( $NaOH\text{NaOH}$ ) solution was neutralised by 20.0 cm³ of dilute sulfuric acid ( $H2SO4\text{H}_2\text{SO}_4$ ). Calculate the concentration of the sulfuric acid in $dm−3\text{mol dm}^{-3}$ .

Write the balanced chemical equation to find the molar ratio.

\begin{aligned} 2\text{NaOH(aq)} + \text{H}_2\text{SO}_4\text{(aq)} \to \text{Na}_2\text{SO}_4\text{(aq)} + 2\text{H}_2\text{O(l)} \end{aligned}

The ratio of $NaOH\text{NaOH}$ to $H2SO4\text{H}_2\text{SO}_4$ is 2 : 1. 2. Calculate the moles of the known substance ( $NaOH\text{NaOH}$ ). First, convert its volume to $dm3\text{dm}^3$ :

\begin{aligned} \text{Volume} &= \frac{25.0}{1000} = 0.025 \text{ dm}^3 \end{aligned}

Then find the moles ( $\times V$ ):

\begin{aligned} \text{Moles of NaOH} &= 0.100 \times 0.025 \\ &= 0.0025 \text{ mol} \end{aligned}

Use the molar ratio to find the moles of the unknown substance ( $H2SO4\text{H}_2\text{SO}_4$ ). Because the ratio is 2 : 1, we divide the moles of $NaOH\text{NaOH}$ by 2:

\begin{aligned} \text{Moles of H}_2\text{SO}_4 &= \frac{0.0025}{2} \\ &= 0.00125 \text{ mol} \end{aligned}

Calculate the concentration of the unknown substance ( $\frac{n}{V}$ ). First, convert its volume to $dm3\text{dm}^3$ :

\begin{aligned} \text{Volume} &= \frac{20.0}{1000} = 0.020 \text{ dm}^3 \end{aligned}

Then calculate the concentration:

\begin{aligned} \text{Concentration} &= \frac{0.00125}{0.020} \\ &= 0.0625 \text{ mol dm}^{-3} \end{aligned}

Converting to grams per cubic decimetre

Sometimes you are asked for the concentration in $dm−3\text{g dm}^{-3}$ instead of $dm−3\text{mol dm}^{-3}$ . This simply tells you what mass of the solute is dissolved in a cubic decimetre of solution.

To convert a concentration from $dm−3\text{mol dm}^{-3}$ to $dm−3\text{g dm}^{-3}$ , multiply the concentration in $dm−3\text{mol dm}^{-3}$ by the relative formula mass ( $M_r$ ) of the substance.

Example

Converting concentration units

Calculate the concentration of $\text{ mol dm}^{-3}$ sulfuric acid ( $H2SO4\text{H}_2\text{SO}_4$ ) in $dm−3\text{g dm}^{-3}$ . (Relative atomic masses: $H=1\text{H} = 1$ , $O=16\text{O} = 16$ , $S=32\text{S} = 32$ )

Calculate the relative formula mass ( $M_r$ ) of sulfuric acid.

\begin{aligned} M_r &= (2 \times 1) + 32 + (4 \times 16) \\ &= 2 + 32 + 64 \\ &= 98 \end{aligned}

Multiply the molar concentration by the $M_r$ to find the mass concentration.

\begin{aligned} \text{Concentration in g dm}^{-3} &= 0.0625 \times 98 \\ &= 6.125 \text{ g dm}^{-3} \end{aligned}

Exam technique

In the exam

When asked to write a method, always state which piece of apparatus you are using for which chemical (e.g., "Use a pipette to measure the alkali").
Don't forget the white tile! It's an easy mark in method questions.
For HT calculations, lay out your working clearly. Even if you make a mistake halfway through (like forgetting to convert $cm3\text{cm}^3$ to $dm3\text{dm}^3$ ), examiners can award error-carried-forward marks if they can read your steps.

Self review

Check yourself

Why is it important to swirl the conical flask during a titration?
What are concordant results?
Which piece of apparatus would you use to measure exactly 25.0 cm³ of a solution?
(HT only) What is the equation linking moles, volume, and concentration?

Titrations (chemistry only)

What you'll learn:

What a titration is and the specialist glassware needed to perform one.
How to carry out a titration accurately (Required Practical 2).
How to select and use concordant results to find a mean titre.
(Higher Tier only) How to calculate unknown concentrations in moles per cubic decimetre and grams per cubic decimetre.

What is a titration?

Definition

Titration

A titration is an experimental technique used to find the exact volume of an acid that neutralises a known volume of an alkali (or vice versa).

By finding these exact reacting volumes, you can figure out the unknown concentration of either the acid or the alkali.

The specialist apparatus

To get highly accurate results, everyday measuring cylinders are not precise enough. We use two specific pieces of glassware: the pipette and the burette.

Pipette: A glass tube with a bulge in the middle, used to measure a single, fixed volume of liquid (usually 25.0 cm³) very accurately. You use a pipette filler to draw the liquid up safely.
Burette: A tall, thin glass tube with a tap at the bottom and markings all the way down. It is used to measure variable volumes drop by drop.

A diagram showing the setup for a titration

Common Mistake

Using Universal Indicator

The method (Required Practical 2)

Use a pipette and pipette filler to add 25.0 cm³ of sodium hydroxide (the alkali) into a clean conical flask.
Add a few drops of a suitable indicator (like phenolphthalein) to the flask and place it on a white tile.
Fill a burette with the hydrochloric acid. Make sure the tap is closed. Remove the funnel from the top of the burette so drops don't fall in later.
Read the initial volume on the burette. Always read from the bottom of the meniscus (the curve of the liquid).
Open the tap to slowly add the acid to the alkali, swirling the flask continuously.
As you get close to the end point, add the acid drop by drop.
Stop adding acid the moment the indicator permanently changes colour (phenolphthalein turns from pink to colourless).
Record the final volume on the burette.
Subtract the initial reading from the final reading to find the titre (the volume of acid added).

Tip

The rough titration

Finding the mean titre

You must repeat the titration until you get concordant results.

Definition

Concordant results

Results that are extremely close together, specifically within 0.10 cm³ of each other.

Once you have at least two concordant results, you ignore the rough titration and any anomalies, and calculate a mean using only those concordant values.

Example

Calculating the mean titre

A student records the following titration results for the volume of acid added: Rough: 23.40 cm³ Run 1: 22.80 cm³ Run 2: 23.10 cm³ Run 3: 22.75 cm³ Run 4: 22.85 cm³ Calculate the mean titre.

Identify the concordant results. We need values that are within 0.10 cm³ of each other. Looking at the data, Run 1 (22.80 cm³), Run 3 (22.75 cm³), and Run 4 (22.85 cm³) are all within 0.10 cm³ of one another.
Discard the non-concordant results. We ignore the Rough run (23.40 cm³) and Run 2 (23.10 cm³).
Calculate the mean of the concordant results by adding them together and dividing by how many there are:

\begin{aligned} \text{Mean} &= \frac{22.80 + 22.75 + 22.85}{3} \\ &= \frac{68.40}{3} \\ &= 22.80 \text{ cm}^3 \end{aligned}

Titration calculations (Higher Tier only)

If you are taking the Higher Tier paper, you will need to use your titration results to calculate the concentration of the unknown solution.

Key Idea

Converting volume units

In chemistry, volumes are usually measured in $cm3\text{cm}^3$ , but formulas need them in $dm3\text{dm}^3$ . To convert from $cm3\text{cm}^3$ to $dm3\text{dm}^3$ , divide by 1000.

Example

Calculating unknown concentration

Write the balanced chemical equation to find the molar ratio.

\begin{aligned} 2\text{NaOH(aq)} + \text{H}_2\text{SO}_4\text{(aq)} \to \text{Na}_2\text{SO}_4\text{(aq)} + 2\text{H}_2\text{O(l)} \end{aligned}

The ratio of $NaOH\text{NaOH}$ to $H2SO4\text{H}_2\text{SO}_4$ is 2 : 1. 2. Calculate the moles of the known substance ( $NaOH\text{NaOH}$ ). First, convert its volume to $dm3\text{dm}^3$ :

\begin{aligned} \text{Volume} &= \frac{25.0}{1000} = 0.025 \text{ dm}^3 \end{aligned}

Then find the moles ( $\times V$ ):

\begin{aligned} \text{Moles of NaOH} &= 0.100 \times 0.025 \\ &= 0.0025 \text{ mol} \end{aligned}

Use the molar ratio to find the moles of the unknown substance ( $H2SO4\text{H}_2\text{SO}_4$ ). Because the ratio is 2 : 1, we divide the moles of $NaOH\text{NaOH}$ by 2:

\begin{aligned} \text{Moles of H}_2\text{SO}_4 &= \frac{0.0025}{2} \\ &= 0.00125 \text{ mol} \end{aligned}

Calculate the concentration of the unknown substance ( $\frac{n}{V}$ ). First, convert its volume to $dm3\text{dm}^3$ :

\begin{aligned} \text{Volume} &= \frac{20.0}{1000} = 0.020 \text{ dm}^3 \end{aligned}

Then calculate the concentration:

\begin{aligned} \text{Concentration} &= \frac{0.00125}{0.020} \\ &= 0.0625 \text{ mol dm}^{-3} \end{aligned}

Converting to grams per cubic decimetre

Example

Converting concentration units

Calculate the relative formula mass ( $M_r$ ) of sulfuric acid.

\begin{aligned} M_r &= (2 \times 1) + 32 + (4 \times 16) \\ &= 2 + 32 + 64 \\ &= 98 \end{aligned}

Multiply the molar concentration by the $M_r$ to find the mass concentration.

\begin{aligned} \text{Concentration in g dm}^{-3} &= 0.0625 \times 98 \\ &= 6.125 \text{ g dm}^{-3} \end{aligned}

Exam technique

In the exam

When asked to write a method, always state which piece of apparatus you are using for which chemical (e.g., "Use a pipette to measure the alkali").
Don't forget the white tile! It's an easy mark in method questions.
For HT calculations, lay out your working clearly. Even if you make a mistake halfway through (like forgetting to convert $cm3\text{cm}^3$ to $dm3\text{dm}^3$ ), examiners can award error-carried-forward marks if they can read your steps.

Self review

Check yourself

Why is it important to swirl the conical flask during a titration?
What are concordant results?
Which piece of apparatus would you use to measure exactly 25.0 cm³ of a solution?
(HT only) What is the equation linking moles, volume, and concentration?

Titrations (chemistry only)

What is a titration?

The specialist apparatus

The method (Required Practical 2)

Finding the mean titre

Titration calculations (Higher Tier only)

Converting to grams per cubic decimetre

You've reached the end

Titrations (chemistry only)

What is a titration?

The specialist apparatus

The method (Required Practical 2)

Finding the mean titre

Titration calculations (Higher Tier only)

Converting to grams per cubic decimetre

You've reached the end

Titrations (chemistry only)

What is a titration?

The specialist apparatus

The method (Required Practical 2)

Finding the mean titre

Titration calculations (Higher Tier only)

Converting to grams per cubic decimetre

You've reached the end

Titrations (chemistry only) Revision Guide

Titrations (chemistry only)

What is a titration?

The specialist apparatus

The method (Required Practical 2)

Finding the mean titre

Titration calculations (Higher Tier only)

Converting to grams per cubic decimetre

You've reached the end

Titrations (chemistry only) Revision Guide