Yield and chemical analysis

Question 1

What is the percentage yield?

Answer 1

What compares your actual yield to your predicted yield

Question 2

How can you calculate the predicted yield of a reaction?

Answer 2

Using the balanced symbol equation

Question 3

What is the formula for %yield?

Answer 3

%Yield = Actual yield (g) / predicted yield (g) X100

Question 4

What does 100% yield mean?

What does 0% yield mean?

Answer 4

All of the reactants formed the expected product

None of the reactants were converted into the product

Question 5

Why is there always a % yield less than 100? (3) Why?

Answer 5

Reversible reactions mean the products react to return to the original reactant, some of the product is always reacting and so there is never a 100% yield (ammonium chloride to ammonia + hydrogen chloride)
Filtration: When you filter the liquid to remove the solid particles you nearly always lose a bit of the liquid or solid
Unexpected reactions that use up the reactions, so less of the product is made

Question 6

Why is percentage yield important for sustainable development?

1. What is sustainable development?
2. So what should be done about % yield?
3. A low yield means ….

Answer 6

Making sure we don’t use resources faster than they can be replaced so there is enough for future generations
Use as little energy as can create the highest % yield obtainable, so that resources are saved
waste chemicals that do not contribute to sustainable development.

Question 7

How can you tell what substances are in a mixture (food colouring), and how are they separated?

Answer 7

Using chromatograhy, a dye or mixture of dyes might be found

Question 8

How can you do the chromatography test? (5)

Answer 8

1. (Extract the colour from a food sample by placing it in a small cup with solvent (water, ethanol salt etc…))
2. Put spots of the coloured solution on the baseline of the filter paper, (draw in pencil)
3. But the paper in a solvent where the baseline is above the solvent.
4. The solvent will seep up the paper taking the dyes with it, different dyes for spots in different places
5. A chromatogram with 4 spots means at least 4 dyes but there could be more with the dyes forming spots in the same place

Question 9

Machines can analyse unknown substances using what methods?

What are the advantages? (3)

Answer 9

Instrumental
It’s very sensitive, the tiniest substances can be detected
It’s very fast and tests can be automated
It’s very accurate

Question 10

What does gas chromatography do?

Answer 10

Identify substances by SEPARATING out a mixture of compounds so they can be identified

Question 11

How does gas chromatography pick out the different substances? (2)

Answer 11

1. A gas is used to carry substances through a column packed with a solid material
2. The substances travel through the tube at different speeds so they’re separated
   The time taken to reach the detector is called the retention time and can be used to identify the substances

Question 12

How does gas chromatography record the different substances? (3)
(What are the axes for the chromatograph?)

Answer 12

The recorder draws a gas chromatograph
The number of peaks show the number of different compounds in the sample
The position of the peaks shows the retention time of each substance
Recorder response, and time in mins along the bottom

Question 13

How does gas chromatography record the different molecules using what? (2)
Link to …. so it can …
You can also work out the …… by …
(What are the axes for the MS?)

Answer 13

The chromatography column can be linked to a mass spectrometer, GC-MS, and can identify the substances leaving the column very accurately
… relative molecular mass of each substance from the graph it draws… reading off the molecular ion peak, (amount (%) and Relative Molecular mass along the bottom)

Question 14

What does mas spectroscopy do? Exam question, 1 mark:

A few possible answers …

Answer 14

Identifies solvents/compounds/substances