Chemistry - How Much?

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Mass Numbers

- The relative mass of protons and neutrons is 1, and the relative mass of an electron is 1/2000.

- The mass number of an atom tells you the total number of protons and neutrons in its nucleus.


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

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Masses of Atoms and Moles

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

-The relative atomic mass (Ar) of an element is usually the same as, or very similar to the mass number of that element. E.g. The relative atomic mass for oxygen is 16.

- We can use the Ar of the various elements to work out the relative formula mass (Mr) of chemical compounds.

E.g. Sodium Chloride

23 + 35.5 = 58.5

Na + Cl = NaCl

- One mole of any substance always contains the same number of particles.

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How to work out percentages of elements in a compound

e.g. Magnesium in magnesium oxide, MgO.

Ar of Mg = 24          Ar, of O = 16

Mr = 40

Mass of magnesium/total mass of compound = 24/40

Percentage of magnesium = 24/40 X 100 = 60%

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Working out the Empirical Formula

e.g. If 9g of aluminium reacts with 35.5g of chlorine, what is the empirical formula of the compound formed?


Divide the mass of each element by its relative atomic mass:

Aluminium = 9/27 = 1/3 mole of aluminium atoms

Chlorine = 35.5/35.5 = 1 mole of chlorine atoms

Ratio: 3 (Cl) : 1 (Al)

Empirical formula: AlCl3.

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Equations and Calculations

- Chemical equations tell us the number of moles of substances in tehh chemical reaction.

- We can use chemical equations to calculate the masses of reactants and products in a chemical reaction from the masses of one mole of each of the substances involved in the reaction.

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Calculating percentage yield

Percentage yield =

amount of product produced/maximum amount of product possible X 100

E.g. Using known masses of A and B, it was calculated that the chemical reaction could produce 2.5g of product C. WHen the reaction is carried out, only 1.5g of C is produced.


1.5/2.5 X 100

= 60%

The percentage yield is 60%

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Atom Economy

- Making as much product as posible means making less waste. It means that as much product as possible is being made from the reactants.

- The amount of the starting materials that end up as useful products is called the atom economy.

E.g. Ethanol can be converted into ethene, which can be used to make poly(ethene)

C²H5OH =               C²H4 +                       H²O

                        Mr Values =       (12 X 2) + (1 X 4)        (1 X 2) + (16 X 1)

                                                         = 28                                 =18

Percentage atom economy = 28/(28 + 18) X 100 = 61%

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Reversible Reactions

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

- In a closed system the rate of the forward and backward reactions are equal at equilibrium.

- Changing the reaction conditions can change the amounts of products and reactants in a reaction mixture.

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Making Ammonia

- Ammonia is an important chemical for making other chemicals, including fertilisers.

- Ammonia is made from nitrogen and hydrogen in the Haber process.

- We carry out the Ahaber process under conditions which are chosen to give a reasonable yield of ammonia as quickly as possible.

- Any unused nitrogen and hydrogen are recycled in the Haber process.


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