A hazard is something that can potentially cause harm to someone or something. A risk is the probability of somebody being harmed when exposed to a hazard.

Before carrying out any experiment, be sure to write a risk assessment which identifies all of the hazards and risks in the experiment.

Standard solutions

Definition

A standard solution is a solution with a known concentration.

To carry out a titration it is important that you make up a standard solution:

procedure

1.	First, calculate the number of moles of solute using: $n \, = \, c \, \times \, V\,(dm^3)$
2.	Next, calculate the mass of solute using: $m \, = n \, \times \, M$
3.	Weigh out the mass of solute using a balance. Be sure to weigh the container first, then add the required mass to this. Note: The balance must have a minimum precision of $2\ d.p.$
4.	Add the solid to a beaker of $100 \, cm^3$ of distilled water and stir to dissolve.
5.	Reweigh the 'empty' container. Take this away from the total measured mass at the start to find the actual mass placed in the beaker.
6.	Use the actual mass and calculate the number of moles using: $n \, = \,\frac{m}{M}$
7.	Calculate the concentration of the standard solution using: $c \, = \, \frac{n}{V \, (dm^3)}$
8.	Place the solution in a volumetric flask. Note: Use a funnel to ensure all of the solution is added.
9.	Rinse the beaker and stirring rod with distilled water and add to the volumetric flask.
10.	Ensure the volumetric flask is filled to the top ( $250 \, cm^3$ ) with distilled water. Note: Make sure the bottom of the meniscus reaches the line on the flask.
11.	Add a stopper and invert the flask a few times to ensure proper mixing.

Titrations

Once you have prepared your standard solution, you can carry out a titration.

procedure

1.	Measure out some of the alkali with an unknown concentration (not the standard solution of acid) using a pipette and add to a conical flask.
2.	Add an indicator to the conical flask.
3.	Rinse the burette with some of the standard solution.
4.	Fill the burette with the standard solution.
5.	Take an initial reading from the burette to see how much acid there is initially.
6.	Complete a rough titration to find the end point. Add the acid from the burette to the alkali in the flask, swirling the flask constantly. Stop adding the acid once there is a colour change.
7.	Record the final reading of acid from the burette.
8.	Do an accurate titration, running the acid within $2 \, cm^3$ from the end point. Then begin to add the acid dropwise slowly.
9.	Calculate the amount of acid needed to neutralise the alkali using a difference in the readings: $Final \space reading \, - \, Inital \space reading \, = \, Titre$
10.	Repeat the experiment until you achieve a set of concordant results. Concordant results are when recordings are within $0.1 \, cm^3$ of each other. Note: Wash the conical flask between each titration to remove the acid and alkali present.
11.	Calculate the mean titre. Be sure to only use your concordant results.

Indicators

Indicators change colour and indicate the end point of a titration due to the change in pH. Two main indicators are used in titrations:

Methyl orange - turns yellow to red upon addition of an acid to alkali
Phenolphthalein - turns red to colourless upon addition of an acid to alkali

Tip: To ensure you are seeing a clear colour change during a titration, place the flask on a white tile.

Concentrations

Unknown concentrations can be calculated using titration reactions.

Note: Always make sure you are carrying out your calculations with the units for volume in $dm^3$ unless the question specifically asks for a volume in $cm^3$ .

Concentration of an unknown alkali solution

procedure

1.	Find the number of moles of the standard solution (acid solution) using: $n \, = \, c \, \times \, V\,(dm^3)$
3.	Write the balanced equation for the reaction to establish the information you know and what you need to work out.
4.	Use the molar ratios to work out the number of moles of the alkali solution.
5.	Calculate the concentration of the alkali solution using: $c \, = \, \frac{n}{V \, (dm^3)}$

Concentration of an unknown acid solution

procedure

1.	Find the number of moles of the alkali dissolved in the solution: $n \, = \, \frac{m}{M}$
2.	Find the concentration of the alkali solution using: $c \, = \, \frac{n}{V \, (dm^3)}$
3.	Use the concentration of the alkali solution along with the mean titre volume to work out the number of moles of the alkali solution using: $n \, = c \, \times \, V \, (dm^3)$
4.	Write the balanced equation for the reaction to establish the information you know and what you need to work out.
5.	Use the molar ratios to work out the number of moles of the acid solution.
6.	Calculate the concentration of the acid solution using: $c \, = \, \frac{n}{V \, (dm^3)}$

Volumes

Similar to unknown concentrations, unknown volumes can be also be calculated.

procedure

1.	First, calculate the number of moles of the standard solution using: $n \, = \, c \, \times \, V\,(dm^3)$
2.	Write the balanced equation for the reaction to establish the information you know and what you need to work out.
3.	Use the molar ratios to work out the number of moles of the solution with the unknown volume.
4.	Use the number of moles and the concentration to calculate the unknown volume using: $V \, (dm^3) \, = \, \frac{n}{c}$

Learn with Basics

Length:

Unit 1

Mole calculations and Avogadro's constant - Higher

Unit 2

Titration calculations - Higher

Jump Ahead

Optional

Unit 3