# xxxx

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Why is the relative atomic mass of chlorine 35.5?
Because it is an average of the isotopes, taking into account the % of each one.
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How do you work out the percentage of an element in a formula?
Find the Mr of the whole thing. Find the Ar of the element you are working out. Divide the Ar by the Mr. Times the answer by 100.
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What is the symbol for a reversible reaction?
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What is a yield?
The amount of product obtained in a reaction.
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Why might you get a low yield?
If the reaction is reversible, so it doesn't go to completion. Some product is lost as it is separated from the reaction mixture during purification. Some reactants react in different ways to make different products.
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What is one way to work out percentage yield?
Percentage yield = Mass you got, divided by the mass you should have got, times 100.
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What is one way to work out percentage yield?
Percentage yield = theoretical mass, divided by the actual mass, times 100.
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How can you identify additives in food?
Chemical analysis.
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What can be used to detect and identify artificial colours?
Paper chromatography
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How would you test for artificial colours?
Put some water in a test tube, and place some chromatography with a pencil baseline, and a spot of the ink you want to test in the centre of the pencil line. Make sure the spot of ink is not in the water.
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How can you tell if a colour is artificial or pure after testing it?
If there is more than one colour on the chromatography paper, it is artificial. If the is only one colour on the chromatography paper, it is pure.
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Why do you draw the baseline in pencil when testing for artificial colours?
Because pencil doesn't doesn't dissolve in water.
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Why would paper chromatography be useful to test food colourings?
To see if the ink is illegal.
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Name 3 ways substances can analysed.
Chemical analysis, paper chromatography, and instrumental analysis.
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What are the advantages instrumental analysis?
It finds out what is in a mixture, it can work with small samples, and it is accurate, sensitive and quick.
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Why does paper chromatography work?
Because some coloured substances are better dissolving in the liquid than they are at bonding with the paper, so they travel further up the paper.
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How does gas chromatography work?
The column is packed with solid material, the sample mixture is carried by mass through the column. Different substances come through at different times.
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In gas chromatography, what does the number of peaks tell you?
The number of substances in something.
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In gas chromatography, what does the position of each peak tell you?
The retention time.
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In gas chromatography, what does the retention time mean?
How long it takes for a substance to travel through the column.
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Name 2 forms of instrumental analysis.
Gas chromatography and a mass spectrometer.
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What is the machine called if gas chromatography and a mass spectrometer are linked?
GC-MS.
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In a mass spectrometer, which peak is the molecular ion peak?
The last peak.
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In a mass spectrometer, what does the molecular ion peak show?
The relative formula mass of that substance.
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Why would you link a mass spectrometer and gas chromatography?
To allow substances in a sample to be separated, then identified.
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What are the advantages of a mass spectrometer?
The mass spectrometer can be used to identify substances quickly and accurately, and in very small amounts. It can also provide the relative formula mass of the substances separated by gas chromatography because of the molecular ion peak.
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## Other cards in this set

### Card 2

#### Front

How do you work out the percentage of an element in a formula?

#### Back

Find the Mr of the whole thing. Find the Ar of the element you are working out. Divide the Ar by the Mr. Times the answer by 100.

### Card 3

#### Front

What is the symbol for a reversible reaction?

What is a yield?

### Card 5

#### Front

Why might you get a low yield?