C2 How much?

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3.1 The mass of atoms

The relative mass of protons and neutrons is 1.

The atomic number of an atom is its number of protons (which equals its number of electrons).

The mass of an atom is the total number of protons and neutrons in its nucleus.

Isotopes are atoms of the same element with different numbers of neutrons.

They have the same chemical properties but different physical properties because of their different masses.

Some isotopes are unstable and radioactive.

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3.2 Masses of atoms and moles

We use relative atomic masses to compare the masses of atoms.

The relative atomic mass of an element is an average value for the isotopes of an elements.

We can compare the masses of atoms by measuring them relative to atoms of carbon-12.

We work out the relative formula mass of a compound by adding up the relative atomic masses of the elements in it, the ratio shown by its formula.

One mole of any substance is its relative formula mass, in grams.

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3.3 Percentages and Formulae

The relative atomic masses of the elements in a compound and its formula can be used to work out its percentage composition.

We can calculate empirical formulae given the masses of percentage composition of elements present.

The empirical formulae is the simplest ratio of the atoms or ions in a compound.

To calculate the empirical formulae of a compound from its percentage composition:

  • divide the mass of each element in 100g of the compound by its atomic mass, to give the ratio of atoms
  • then convert this to the simplest whole number ratio

What is the empirical formula of the compound that contains 70% iron and 30% oxygen?

Mass in 100g of compound: Iron - 70    Oxygen - 30

Ratio of atoms or moles of atoms: Iron - 70/56 = 1.25   Oxygen - 30/16 = 1.875

Simplest ratio of atoms (/ by smallest): 1.25/1.25 = 1   1.875/1.25 = 1.5: Fe2O3

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3.4 Equations and Calculations

Balanced symbol equations tell us the number of moles of substances involved in a chemical reaction.

We can use balanced symbol equations to calculate the masses of reactants and products in a chemical reaction.


Calculate the mass of calcium oxide that can be made from 10g of calcium carbonate in the reaction - 

CaCO3 --> CaO + CO2

CaCO3 = 100, one mole of CaCO3 gives one mole of CaO so 40 + 16 = 56

For 10g, (10/100) x 56 = 5.6 g

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3.5 The yield of a chemical reaction

The yield of chemical reaction is how much product is made.

The percentage yield tells us how much product is made compared with the maximum amount that could be made.

Percentage yield =  amount of product collected                                              maximum amount of product possible   x 100%

Factors affecting the yield of a chemical reaction include product being left behind in the apparatus and difficulty separating the products from the reaction mixture.

It is important to maximise yield and minimise energy wasted to conserve the Earth's limited resources and reduce pollution.

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3.6 Reversible Reactions

In a reversible reaction the products of the reaction can react to make the original reactants.

We can show a reversible reaction using the reversible reaction sign:

Thermal decomposition of ammonium chloride is a reversible reaction:

ammonium chloride    ammonia + hydrogen chloride

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3.7 Analysing substances

Chemical analysis is used to identify food additives.

Additives may be added to food in order to improve its appearance, taste and shelf life.

Food scientists can analyse foods to identify additives e.g. by using paper chromatography.

Modern instrumental techniques provide fast, accurate and sensitive ways of analysing chemical substances.

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3.8 Instrumental Analysis

Compounds in a mixture can be separated using gas chromatography.

Once separated, compounds can be identified using a mass spectrometer.

The mass spectrometer can be used to find the relative molecular mass of a compound from its molecular ion peak.

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