C2.3

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What are isotopes?
All atoms of an element have the same atomic number (protons + electrons) but those with diferebt numbers of neutrons are called isotopes.
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What is Ar?
Relative atomic masses are used as atoms are too small to weigh – Ar of element in grams = mole
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How do scientists know the Ar of an element?
It is often the same as the mass number of the main isotope of the element.
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What is the Mr of a substance?
Relative formula mass is found by adding up the relative atomic masses of the atoms in its formula.
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How can we calculate the percentage of any of the elements in a compound?
Ar of element ÷ Mr of the compound × 100
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What is empirical formula?
The simplest ratio of the atoms/ions in a compound – formula used for ionic compounds but isn’t always the same as molecular formula for covalent compounds.
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How can we calculate the empirical formula of a compound from its percentage compositions?
Mass of each element in 100g ÷ Ar then convert this to the simplest whole number ratio.
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Why is it not usually possible to get 100% yield from a chemical reaction?
Reactions may not go to completion, other reactions may happen, some product may be lost when separated/collected
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Why should chemical manufacturers use reactions with high yields?
To help conserve resources, reduce waste and/or pollution
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What methods can be used to analyse the colours in food and why are they used?
Paper chromatography and mass spectrometry are used to ensure only safe, permitted additives are used.
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How is paper chromatography used and why?
To analyse artificial colours in food. A spot of colour is put onto paper and a solvent is allowed to move through the paper. The colours move different distances depending on their solubility.
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What can gas chromatography linked to mass spectrometry do?
Analyse mixtures that need to be separated so that the compound can be identified.
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How is gas chromatography carried out?
The mixture is carried by a gas through a long column packed with particles of a solid and individual compounds travel though the column at different speeds and so come out different times.
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What is recorded in GC-MS analysis?
The amount of substance leaving the column at different times is recorded against time and shows the number of compounds in the mixture and their retention times.
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What can the retention times of output from the gas chromatography column be compared with?
The results for known compounds to help identify the compounds in the mixture and be linked to a mass spectrometer (GC-MS) to give the relative molecular mass of a compound.
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How is the relative molecular shown in a mass spectrum?
From the molecular ion peak or the peak with the largest mass (furthest to the right in the spectrum)
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Card 2

Front

What is Ar?

Back

Relative atomic masses are used as atoms are too small to weigh – Ar of element in grams = mole

Card 3

Front

How do scientists know the Ar of an element?

Back

Preview of the front of card 3

Card 4

Front

What is the Mr of a substance?

Back

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Card 5

Front

How can we calculate the percentage of any of the elements in a compound?

Back

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