4.2.5a Titrations

Titration finds the exact volumes that react

Definition

Titration

A technique used to determine an unknown concentration by reacting a measured volume of one solution with a solution of known concentration.

Definition

Indicator

A substance that changes colour over a particular pH range and can show when the end point of a titration has been reached.

A titration measures the volume of acid that exactly reacts with a fixed volume of alkali.
It is used for strong acids and strong alkalis, such as hydrochloric, sulfuric or nitric acid with sodium or potassium hydroxide.
A suitable single-colour-change indicator, such as phenolphthalein or methyl orange, shows when the reaction is complete.
Universal indicator is unsuitable because its gradual range of colours makes the exact change hard to judge.

Practical

Investigation: finding the volume of acid that neutralises 25 $cm3\text{cm}^3$ of alkali

Use a volumetric pipette and filler to run exactly 25.0 $cm3\text{cm}^3$ of sodium hydroxide solution into a clean conical flask.
Add a few drops of phenolphthalein, which turns the alkali pink, and stand the flask on a white tile so the colour is easy to see.
Fill a burette with the dilute acid, run a little through the tap to remove air bubbles, and read the initial volume at eye level from the bottom of the meniscus.
Run the acid in quickly at first while swirling the flask, then add it drop by drop as the pink colour starts to fade.
Stop the moment one drop turns the solution permanently colourless, and record the final burette reading.
Work out the titre by subtracting the initial reading from the final reading, recorded to the nearest 0.05 $cm3\text{cm}^3$ .
Do a rough titration first, then repeat carefully until two titres agree within 0.10 $cm3\text{cm}^3$ , and take the mean of those concordant results.
Wear eye protection, because both the acid and the alkali are irritant.

Careful readings and concordant titres give reliable results

Definition

End point

The stage in a titration when the indicator shows its first permanent colour change.

Definition

Titre

The volume of solution delivered from a burette, calculated by subtracting the initial burette reading from the final reading.

The end point is the first permanent colour change, from pink to colourless with phenolphthalein.
Read the burette at eye level from the bottom of the meniscus to avoid a parallax error.
Make sure there is no air bubble in the burette tip, or the titre will be too large.
Concordant titres agree within 0.10 $cm3\text{cm}^3$ , and only these are used to calculate the mean.
The rough titre is never included in the mean.

Common Mistake

Do not use too much indicator, because it can slightly change the volume needed.
Do not stop at a colour change that fades again when you swirl the flask.
Do not include the rough titre in your mean.

Why the apparatus is chosen

The pipette delivers one accurate fixed volume of alkali.
The burette measures the changing volume of acid to the nearest 0.05 $cm3\text{cm}^3$ .
A measuring cylinder is not precise enough for either volume in a titration.
Repeating until titres are concordant reduces the random error in the result.

Self review

What does a titration measure?
Why is universal indicator unsuitable for a titration?
What is the colour change of phenolphthalein at the end point?
How do you work out a titre from two burette readings?
Which titres are used when calculating the mean?

4.2.5b Calculating quantities in titrations

Titration calculations link volumes through moles

Definition

Mole

The amount of a substance, measured in mol, where one mole has a mass in grams numerically equal to its relative formula mass.

Definition

Concentration

The amount of solute dissolved in a given volume of solution, often measured in mol/dm3 or g/dm3.

Start with the reactant whose concentration and volume are both known.
Find its moles using $cm31000\text{moles} = \dfrac{\text{concentration} \times \text{volume in cm}^3}{1000}$
Use the ratio in the balanced equation to find the moles of the other reactant.
Find the unknown concentration using $cm3\text{concentration} = \dfrac{\text{moles} \times 1000}{\text{volume in cm}^3}$

Key Idea

Always convert volumes from $cm3\text{cm}^3$ to $dm3\text{dm}^3$ , or keep the $÷1000\div 1000$ in the formula.
The mole ratio comes from the balanced equation, so balance it first.

Worked example: the concentration of sulfuric acid

Definition

Balanced equation

A chemical equation containing the same number of atoms of each element on both sides.

In a titration, 24.0 $cm3\text{cm}^3$ of sulfuric acid exactly neutralises 25.0 $cm3\text{cm}^3$ of 0.100 $mol/dm3\text{mol/dm}^3$ sodium hydroxide, with equation $2NaOH+H2SO4→Na2SO4+2H2O2\text{NaOH} + \text{H}_2\text{SO}_4 \rightarrow \text{Na}_2\text{SO}_4 + 2\text{H}_2\text{O}$
Moles of $mol\text{NaOH} = \dfrac{0.100 \times 25.0}{1000} = 0.00250\ \text{mol}$ .
The ratio is $2 : 1$ , so moles of $mol\text{H}_2\text{SO}_4 = \dfrac{0.00250}{2} = 0.00125\ \text{mol}$ .
Concentration of $mol/dm3\text{H}_2\text{SO}_4 = \dfrac{0.00125 \times 1000}{24.0} = 0.0521\ \text{mol/dm}^3$ .

Exam technique

Set the work out in clear steps, because each stage earns method marks even if a later number is wrong.
Use the mean concordant titre, not the rough titre, as the volume in the calculation.

Converting between $mol/dm3\text{mol/dm}^3$ and $g/dm3\text{g/dm}^3$

Definition

Solute

The substance dissolved in a solvent to form a solution.

Convert using $mol/dm3×Mr\text{concentration in g/dm}^3 = \text{concentration in mol/dm}^3 \times M_r$
For sulfuric acid $M_r = 98$ , so $g/dm30.0521\ \text{mol/dm}^3 \times 98 = 5.11\ \text{g/dm}^3$ .
Reverse the conversion by dividing the $g/dm3\text{g/dm}^3$ value by the $M_r$ .

Self review

Write the formula linking moles, concentration and volume in $cm3\text{cm}^3$ .
In $2NaOH+H2SO42\text{NaOH} + \text{H}_2\text{SO}_4$ , how many moles of acid react with 0.00250 mol of alkali?
How do you convert a concentration in $mol/dm3\text{mol/dm}^3$ to $g/dm3\text{g/dm}^3$ ?
Why should you use the mean concordant titre in the calculation?
Find the concentration of an acid if 0.00200 mol reacts in 25.0 $cm3\text{cm}^3$ of solution.

4.2.5a Titrations

Titration finds the exact volumes that react

Definition

Titration

A technique used to determine an unknown concentration by reacting a measured volume of one solution with a solution of known concentration.

Definition

Indicator

A substance that changes colour over a particular pH range and can show when the end point of a titration has been reached.

A titration measures the volume of acid that exactly reacts with a fixed volume of alkali.
It is used for strong acids and strong alkalis, such as hydrochloric, sulfuric or nitric acid with sodium or potassium hydroxide.
A suitable single-colour-change indicator, such as phenolphthalein or methyl orange, shows when the reaction is complete.
Universal indicator is unsuitable because its gradual range of colours makes the exact change hard to judge.

Practical

Investigation: finding the volume of acid that neutralises 25 $cm3\text{cm}^3$ of alkali

Use a volumetric pipette and filler to run exactly 25.0 $cm3\text{cm}^3$ of sodium hydroxide solution into a clean conical flask.
Add a few drops of phenolphthalein, which turns the alkali pink, and stand the flask on a white tile so the colour is easy to see.
Fill a burette with the dilute acid, run a little through the tap to remove air bubbles, and read the initial volume at eye level from the bottom of the meniscus.
Run the acid in quickly at first while swirling the flask, then add it drop by drop as the pink colour starts to fade.
Stop the moment one drop turns the solution permanently colourless, and record the final burette reading.
Work out the titre by subtracting the initial reading from the final reading, recorded to the nearest 0.05 $cm3\text{cm}^3$ .
Do a rough titration first, then repeat carefully until two titres agree within 0.10 $cm3\text{cm}^3$ , and take the mean of those concordant results.
Wear eye protection, because both the acid and the alkali are irritant.

Careful readings and concordant titres give reliable results

Definition

End point

The stage in a titration when the indicator shows its first permanent colour change.

Definition

Titre

The volume of solution delivered from a burette, calculated by subtracting the initial burette reading from the final reading.

The end point is the first permanent colour change, from pink to colourless with phenolphthalein.
Read the burette at eye level from the bottom of the meniscus to avoid a parallax error.
Make sure there is no air bubble in the burette tip, or the titre will be too large.
Concordant titres agree within 0.10 $cm3\text{cm}^3$ , and only these are used to calculate the mean.
The rough titre is never included in the mean.

Common Mistake

Do not use too much indicator, because it can slightly change the volume needed.
Do not stop at a colour change that fades again when you swirl the flask.
Do not include the rough titre in your mean.

Why the apparatus is chosen

The pipette delivers one accurate fixed volume of alkali.
The burette measures the changing volume of acid to the nearest 0.05 $cm3\text{cm}^3$ .
A measuring cylinder is not precise enough for either volume in a titration.
Repeating until titres are concordant reduces the random error in the result.

Self review

What does a titration measure?
Why is universal indicator unsuitable for a titration?
What is the colour change of phenolphthalein at the end point?
How do you work out a titre from two burette readings?
Which titres are used when calculating the mean?

4.2.5b Calculating quantities in titrations

Titration calculations link volumes through moles

Definition

Mole

The amount of a substance, measured in mol, where one mole has a mass in grams numerically equal to its relative formula mass.

Definition

Concentration

The amount of solute dissolved in a given volume of solution, often measured in mol/dm3 or g/dm3.

Start with the reactant whose concentration and volume are both known.
Find its moles using $cm31000\text{moles} = \dfrac{\text{concentration} \times \text{volume in cm}^3}{1000}$
Use the ratio in the balanced equation to find the moles of the other reactant.
Find the unknown concentration using $cm3\text{concentration} = \dfrac{\text{moles} \times 1000}{\text{volume in cm}^3}$

Key Idea

Always convert volumes from $cm3\text{cm}^3$ to $dm3\text{dm}^3$ , or keep the $÷1000\div 1000$ in the formula.
The mole ratio comes from the balanced equation, so balance it first.

Worked example: the concentration of sulfuric acid

Definition

Balanced equation

A chemical equation containing the same number of atoms of each element on both sides.

In a titration, 24.0 $cm3\text{cm}^3$ of sulfuric acid exactly neutralises 25.0 $cm3\text{cm}^3$ of 0.100 $mol/dm3\text{mol/dm}^3$ sodium hydroxide, with equation $2NaOH+H2SO4→Na2SO4+2H2O2\text{NaOH} + \text{H}_2\text{SO}_4 \rightarrow \text{Na}_2\text{SO}_4 + 2\text{H}_2\text{O}$
Moles of $mol\text{NaOH} = \dfrac{0.100 \times 25.0}{1000} = 0.00250\ \text{mol}$ .
The ratio is $2 : 1$ , so moles of $mol\text{H}_2\text{SO}_4 = \dfrac{0.00250}{2} = 0.00125\ \text{mol}$ .
Concentration of $mol/dm3\text{H}_2\text{SO}_4 = \dfrac{0.00125 \times 1000}{24.0} = 0.0521\ \text{mol/dm}^3$ .

Exam technique

Set the work out in clear steps, because each stage earns method marks even if a later number is wrong.
Use the mean concordant titre, not the rough titre, as the volume in the calculation.

Converting between $mol/dm3\text{mol/dm}^3$ and $g/dm3\text{g/dm}^3$

Definition

Solute

The substance dissolved in a solvent to form a solution.

Convert using $mol/dm3×Mr\text{concentration in g/dm}^3 = \text{concentration in mol/dm}^3 \times M_r$
For sulfuric acid $M_r = 98$ , so $g/dm30.0521\ \text{mol/dm}^3 \times 98 = 5.11\ \text{g/dm}^3$ .
Reverse the conversion by dividing the $g/dm3\text{g/dm}^3$ value by the $M_r$ .

Self review

Write the formula linking moles, concentration and volume in $cm3\text{cm}^3$ .
In $2NaOH+H2SO42\text{NaOH} + \text{H}_2\text{SO}_4$ , how many moles of acid react with 0.00250 mol of alkali?
How do you convert a concentration in $mol/dm3\text{mol/dm}^3$ to $g/dm3\text{g/dm}^3$ ?
Why should you use the mean concordant titre in the calculation?
Find the concentration of an acid if 0.00200 mol reacts in 25.0 $cm3\text{cm}^3$ of solution.

4.2.5 Titrations (chemistry only)

4.2.5a Titrations

Titration finds the exact volumes that react

Investigation: finding the volume of acid that neutralises 25 $cm3\text{cm}^3$ of alkali

Careful readings and concordant titres give reliable results

Why the apparatus is chosen

4.2.5b Calculating quantities in titrations

Titration calculations link volumes through moles

Worked example: the concentration of sulfuric acid

Converting between $mol/dm3\text{mol/dm}^3$ and $g/dm3\text{g/dm}^3$

Review 4.2.5 Titrations by teaching Genie

Recap your knowledge with an interactive lesson

Remember key concepts with flashcards

4.2.5 Titrations (chemistry only)

4.2.5a Titrations

Titration finds the exact volumes that react

Investigation: finding the volume of acid that neutralises 25 $cm3\text{cm}^3$ of alkali

Careful readings and concordant titres give reliable results

Why the apparatus is chosen

4.2.5b Calculating quantities in titrations

Titration calculations link volumes through moles

Worked example: the concentration of sulfuric acid

Converting between $mol/dm3\text{mol/dm}^3$ and $g/dm3\text{g/dm}^3$

Review 4.2.5 Titrations by teaching Genie

Recap your knowledge with an interactive lesson

Remember key concepts with flashcards

4.2.5 Titrations (chemistry only)

4.2.5a Titrations

Titration finds the exact volumes that react

Investigation: finding the volume of acid that neutralises 25 cm3\text{cm}^3cm3 of alkali

Careful readings and concordant titres give reliable results

Why the apparatus is chosen

4.2.5b Calculating quantities in titrations

Titration calculations link volumes through moles

Worked example: the concentration of sulfuric acid

Converting between mol/dm3\text{mol/dm}^3mol/dm3 and g/dm3\text{g/dm}^3g/dm3

Review 4.2.5 Titrations by teaching Genie

Recap your knowledge with an interactive lesson

Remember key concepts with flashcards

4.2.5 Titrations Revision Guide

4.2.5 Titrations (chemistry only)

4.2.5a Titrations

Titration finds the exact volumes that react

Investigation: finding the volume of acid that neutralises 25 cm3\text{cm}^3cm3 of alkali

Careful readings and concordant titres give reliable results

Why the apparatus is chosen

4.2.5b Calculating quantities in titrations

Titration calculations link volumes through moles

Worked example: the concentration of sulfuric acid

Converting between mol/dm3\text{mol/dm}^3mol/dm3 and g/dm3\text{g/dm}^3g/dm3

Review 4.2.5 Titrations by teaching Genie

Recap your knowledge with an interactive lesson

Remember key concepts with flashcards

4.2.5 Titrations Revision Guide

Investigation: finding the volume of acid that neutralises 25 $cm3\text{cm}^3$ of alkali

Converting between $mol/dm3\text{mol/dm}^3$ and $g/dm3\text{g/dm}^3$

Investigation: finding the volume of acid that neutralises 25 $cm3\text{cm}^3$ of alkali

Converting between $mol/dm3\text{mol/dm}^3$ and $g/dm3\text{g/dm}^3$