C2 3.2 How Much?

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Masses of atoms and moles

Relative atomic mass (Ar) is the mass of an atom, for example carbon = 12.

The relative formula mass (Mr) of a substance is the sum of all the relative atomic masses in the compound. For example, H20 = (2x1)+16 = 18

The relative atomic mass of an element in grams is called one mole of the element. For example, one mole of NaOH - Ar of Na = 23, O = 16, H = 1. 23+16+1 = 40g

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Percentages and Formulae

What is the percentage of carbon in carbon dioxide, C02? 

Ar of C = 12, 0 = 16

Mr of C02 = 12 + (16x2) = 44

12/44 x 100 = 27.3%

The empirical formula is the simplest ratio of atoms in a compound. (It is not always the same as the molecular formula).

What is the empirical formula of the hydrocarbon that contains 80% carbon?

1) Mass in 100g of compund - Carbon = 80g,  Hydrogen = 20g

2) Mass/Ar - 80/12 = 6.67,  20/1 = 20

3) Divide by smallest number - 6.67/6.67 = 1,  20/6.67 = 3

4) Ratio - 1:3

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Equations and Calculations

Moles =  Mass / Relative Atomic Mass          n = m / Mr        

What mass of carbon dioxide is produced when 5.6g of butene (C4H8) is burnt?

1              :          4

C4H8 + 6O2 -> 4CO2 + 4H20

1. n = m / mr  n = 5.6g / 56 = 0.1

2. Ratio  = 1:4

3. 0.1 x 4

4. n = m / mr   0.4 = m / 44   0.4x44 = 17.6g

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The Yield of a Chemical Reaction

Percentage Yield

Percentage Yield = (amount of product collected / maximum amount of product possible) x 100%

The maximum amount of product possible is calculated from the balanced equation for the reaction. 

A student collected 2.3g of magnesium oxide from 2.0g magnesium.

Theoretically: 2Mg + O2 -> 2MgO, so 48g of Mg should give 80g of MgO, and so 2.0g of Mg should give 2 x 80/48 = 3.33g of MgO.

Percentage Yield = (2.3/3.33) x 100 = 69%

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Reversible Reactions

If the products of a chemical reaction can react to produce the reactants, the reaction can go in both directions.

This type of reaction is called a reversible reaction and represented with the symbol Equilibrium symbol (http://www.bbc.co.uk/staticarchive/b65c7d8679f3cb0815e4693605278d4d44cd76d7.gif)

E.g. ammonium chloride Equilibrium symbol (http://www.bbc.co.uk/staticarchive/b65c7d8679f3cb0815e4693605278d4d44cd76d7.gif) ammonia + hydrogen chloride 

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Analysing Substances & Instrumental Analysing

Chromatography can be used to separate mixtures of coloured compounds.

Simple chromatography is carried out on paper. A spot of the mixture is placed near the bottom of a piece of chromatography paper and the paper is then placed upright in a suitable solvent, eg water. As the solvent soaks up the paper, it carries the mixtures with it. Different components of the mixture will move at different rates. This separates the mixture out.

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. The peak furthest to the right in a mass spectrum is called the 'molecular ion peak'. Its relative mass is the relative formula mass of the substance being analysed.

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