If you know the actual amounts of two substances that react, you can;

-predict other amounts that will react

-say how much product will form

 

So before calculating, you should know these two laws of chemistry

ELEMENTS ALWAYS REACT IN THE SAME RATIO, TO FORM A GIVEN COMPOUND

THE TOTAL MASS DOES NOT CHANGE DURING A CHEMICAL REACTION

for example, if you combine 

 

24g of Magnesium and 16g of Oxygen 

you will also get the same amount of magnesium and oxygen in the final product

 

 

another example is when 

128g of Iron Oxide + 12g of Carbon

you will get 

128g of Iron + 12g of Carbon and 24g of oxygen

 

 

 

 

 

 

Calculating Quantities

 

To make calculations easy, just remember the laws above

So 24g of Magnesium reacts with 16g of Oxygen to give off Magnesium Oxide

What amount of oxygen will react with 48g of magnesium?

 

simply divide 48 by 24,

then multiply 16g by the answer

your answer will be;

32g of oxygen will react with 48g of magnesium

 

Now if you want to find the mass of the final product, simply add the mass of magnesium and oxygen

24g + 16g = 40g

 

So by the same method above, you would be able to find the amount of magnesium oxide formed if there was only

8g of oxygen

 

divide 16 by 8

then divide 24 (amount of magnesium) by the answer

you will get;

oxygen: 16 / 8 = 2

magnesium: 24 / 2 = 12

 

when you have found both masses, add them together and you've found the mass of the magnesium oxide produced

12g + 2g = 14g (magnesium oxide)

 

 

 

 

 

Percentage of composition 

 

Here's a reminder:

-the full amount of anything is 100 %

-change a fraction to a % by multipying it by 100

 

Here's an example;

To find the percentage of magnesium in magnesium oxide,

divide the mass of magnesium by the Mr of magnesium oxide (compound)

and multiply it by 100

 

24 / 40  x 100 = 60 %

 

 

 

 

 

Calculating % purity

 

The percentage of purity can be found by using this formula

 

                mass of pure substance in it     x 100 

total mass

 

For example,

if there 18 tonnes of copper gave 16 tonnes of pure copper upon refining,

what is the percantage of purity?

 

                    16 tonnes        x 100 %

18 tonnes

 

% of purity of copper = 88.89%

 

 

Here's another example

If 60 tonnes of magnesium gave out 42 tonnes of pure magnesium,

the purity would be;

 

                  42 tonnes       x 100 %

60 tonnes

 

% of purity of magnesium = 70 % 

 

 

 

 

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