topic 6

Relative Atomic Mass = Ar

• Ar = Mass number
• mass number is the number is the bigger number on the periodic table (C carbon = 12)

Relative Formula Mass = Mr

• Mr = in a compound

1) multiply the Ar of the different elements to the amount of compounds

2) add the answers together

3) your answer will be a number

Calculating the % composition of Elements in a compound

• FORMULA = [ Ar × No. of atoms (of that element) ÷ Mr (of whole compound) ]  × 100
• the answer is a percentage %

The Empirical Formula (from masses)

1) List all the elements (in the compound)

2) then write beneath them their experimental masses.

3) divide each mass by the Ar for that particular element

4) turn the numbers into a ratio (by multiplying or dividing them)

5) put the ratio in it's simplest form (this tells you the empirical formula of the compound)

this is the only way of finding the formula of a compound

Calculating masses in a reaction

1) write out the balanced equation

2) work out the Mr

£) apply the rule : ÷ to get one and × to get all    ( apply this to the first substance you are given info about and then the other one)

Example:

1) 2Mg + O2  →   2MgO

2) 2 × 24  →   2 × 24 (24 + 16)                                                                                                48    →       80

3) 48 and 80, tell us that 48g of Mg react to give 80g of MgO

       48g of Mg..........reacts to give............80g og MgO                                                            1g of Mg ..........reacts to give............                                                                               60g of Mg ..........reacts to give.............

48g of Mg.......... 80g of MgO                                                                                               1g of Mg........... 1.67g of MgO                                                                                               60g of MgO.........100g of MgO

60g of magnesium will produce 100g of magnesium oxide.

The mass of a product is called the yield of a reaction

masses you calculate in this way are called theoretical yields

in practice you never get 100% of of the yeild, so the amount of product will be less than calculated.

percentage yield

• the yield of a reaction is the mass of product it produces
• the more reactants you start with the higher the actual yeild will be
• work out the theoretical (predicted) yeild of a reaction from the balanced equation

Percentage Yeild FORMULA = [actual yeild (grams) ÷ theoretical yield (grams)] ×  100

percentage yeild;

• always between 0 and 100%
• a 100% yield means you got all the product you expected to get
• a     0% yield means no reactants were converted into product/ no product made

  you never get a 100% yield, some product or reactant is always lost.

1) Incomplete reactions

if not all the reactants are converted into a product the reaction is incomplete.                     the reaction is incomplete and the yield will b lower than expected

2) practical losses during preparation

when you transfer chemicals from one container to another, some of it is left behind on the inside surface of the old container

3) Unwanted reactions

Sometimes unexpected reactions occur, so the yield of the expected reaction reduces. This can be caused by impurities in the reactants. Or a change in the reaction conditions affects what products you make.

Waste in reactions costs money

• Reactions that make waste are not profitable, because waste products are not commercially useful
• a harmful waste product is a threat to people and the environment
• disposing of harmful waste products safely can be very expensive

Reactions that produce products in a way that safely makes the most profit:

1) they have a high percentage yield ( so that lots of product is made from expensive raw materials)

2) All of the products are commercially useful so there isn't any waste

3) They are a suitable speed (made quickly and safely)