Purity, Formulations & Chromatography (AQA GCSE Chemistry)

Methylated spirit is a versatile product made from a mixture of substances.

Table 1 below shows the mass of substances present in a sample of methylated spirit.

Table 1

What name is given to a product such as methylated spirit?

Calculate the percentage of ethanol in the sample of methylated spirit.

Use Table 1 above to answer this question.

Give your answer to 2 significant figures. 

The sample of methylated spirit was analysed and found to contain small amounts of other substances not stated in the formulation.

Comment on how this may affect the performance of the formulation.

Formulations are used in many different industries. Give two examples of formulations.

Describe the design and general properties of a formulation. 

Include some examples of common formulations.

A student carried out chromatography on a sample of green ink. 

The distance moved by the solvent front is 5.8 cm. 

The distance moved by the yellow dye is 4.5 cm.

Calculate the R_f value of the yellow dye to 2 significant figures. 

A student was investigating the colours in black food colouring. 

Chromatography was carried out on samples A-E and compared to the black food coloring. 

The following chromatogram was produced. 

Which of the following conclusions is not true for this chromatogram?

This question is about pure substances and mixtures.

Explain what the term ‘pure’ means in chemistry and compare it to the everyday meaning.

Explain how boiling point and melting point analysis can be used to differentiate between pure and impure substances.

A student was testing the melting point of a sample of gallium using a melting point apparatus. 

The sample started to melt at 24 ^oC and melted completely at 27 ^oC. 

Data tables places the melting point of gallium at 29.76 ^oC

Is the sample of gallium pure or impure? 

Give reasons for your answer.

This question is about chromatography.

A student used paper chromatography to investigate different types of food dye.

This is the method the student used.

1. Put a spot of food colouring brand X on the start line.
2. Place spots of four separate dyes labelled A, B, C and D, on the start line.
3. Place the bottom of the paper in a beaker of water and leave it there for several minutes.

The apparatus the student used is shown in Figure 1.

Figure 1

Identify two mistakes the student made in setting up the apparatus and explain what problems the mistakes would cause.

Explain how the student could avoid these mistakes.

Another student set up the apparatus correctly and obtained a set of results which are shown in Figure 2. 

The result for sample D is not shown.

Figure 2

Calculate the R_f value of dye B, showing clearly your working out.

Give your answer to two significant figures.

The  R_f value of Dye D = 0.80. 

Use this value to calculate the distance dye D moved on the chromatography paper.

Explain how chromatography is able to separate the different dyes in sample X.

A chemist was asked to analyse several samples of substances used as food colourings. 

He performed paper chromatography on the samples he was given, which he labelled A, B, C, D and E.

The chromatogram in Figure 3 shows the chemists' results.

Figure 3

What can you deduce about the composition of the black food colouring?

Calculate the R_f value of dye C, showing clearly your working out.

Give your answer to two significant figures.

Paracetamol is a formulation.

One tablet has a mass of 1.30 g and contains 500 mg of the active ingredient, paracetamol. 

What is the percentage by mass of paracetamol in one tablet?

A student set up the following apparatus to investigate the colours in three different food colourings.  

What are the stationary and mobile phases for this experiment? 

Black ink is made from other colour dyes.

A student investigated black ink using paper chromatography in a beaker with water as the solvent.

Figure 1 shows the chromatogram that the student obtained. It is not to scale.

Figure 1

The R_f value of the blue dye = 0.86
 
The distance moved by the solvent = 6.3 cm
 
Calculate the distance moved by the blue dye, in cm.
 
Give your answer to two significant figures.

The student took measurements from the chromatogram and calculated the R_f value of the red dye to be 1.3.

Explain why this value is not correct.

What does the R_f value of a dye depend on?

In Figure 1, the yellow dye has not moved from the start line.

Give a reason why it has not moved and suggest how the method could be altered to obtain a chromatogram in which all three dyes move up the paper.

Paraffin wax is a flammable substance that is obtained from crude oil.

A student investigated how the temperature of paraffin wax changed as it cooled using the steps below.

• Heat the paraffin wax until it melts
• Continue to heat for a further minute
• Measure and record the temperature every minute

The results they obtained are plotted in Figure 1.

Figure 1

What conclusion can you draw from these results?

Explain your answer.

Suggest a method that would be most suitable for heating the wax.

The student repeated the experiment using pure stearic acid which has a melting point of 69 ^oC.
 
They heated the sample up to 80 ^oC and allowed it to cool. After 30 minutes, the temperature of the stearic acid was 40 ^oC.
 
Sketch the expected results in Figure 2.

A student investigated the food colouring in three drinks using paper chromatography.

Figure 1 shows the chromatogram showing the student's results.

Figure 1

Why does the start line need to be drawn in pencil? 

What conclusions can the student make from the chromatogram in Figure 1?

Explain how the student can calculate the R_f value of the colours contained in the drinks.

Explain how chromatography separates the different colours in the drinks by paper chromatography.

Cold and flu medicines are formulations.

An example of the packaging of some tablets used to treat cold and flu is shown in Figure 1.

Figure 1

State what a formulation is, and explain how the information on the packet of the cold and flu tablets in Figure 1 shows that it is a formulation.
 
In your answer, suggest possible reasons for the other, non-active, ingredients.

Using the information in Figure 1, calculate the percentage by mass of phenylephrine hydrochloride in each tablet.

The formula of paracetamol is C₈H₉NO₂.

Calculate the relative formula mass of paracetamol. 

Relative atomic masses:  H = 1;  C = 12;  N = 14;  O = 16 

New evidence suggests that a food colouring that has a R_f value of 0.7 may cause dangerous side effects.

The chromatogram of two sweets, X and Y, were collected and the results are shown in Figure 1.

Figure 1

For each of the sweets, state whether they should or should not be sold until further research into the food colouring has taken place.
 
Give reasons for your answer.
 
Use the scale on the chromatogram to help you answer this question.

Some food colouring additives are thought to cause hyperactivity in young children.

Suggest two reasons why manufacturers still use these additives in their products.

One of the additives thought to cause hyperactivity is the additive tartrazine, E102, and it is often added to orange drinks.

An investigation into the food colouring in an orange drink was carried out using paper chromatography.

Figure 2 shows the chromatogram produced.

 
Figure 2
 

 
The manufacturer of the drink stated that their drink did not contain any additives that caused hyperactivity.

Is this claim correct? Explain your answer.

Use the chromatogram in Figure 2.