CALCULATIONS FROM EQUATIONS, using moles

 

no. of moles = mass in g

                   

                       molar mass 

 

What can an equation tell you?

An equation of a reaction can show how many moles of each substance that takes part and also how many grams that take part.

 

For example,

Hydrogen + Oxygen --> Water

 

Firstly, write an equation and make it balanced.

 

H2 + O2 ---> H2O 

 

2H2 + O2 ---> 2H2O

 

This can be written as 2 molecules of Hydrogen reacts with 1 molecule of Oxygen creates 2 molecule of water.

 

or

 

2 mole of Hydrogen molecules react with 1 mole of Oxygen molecules to give 2 mole of water molecules.

 

Then,the next step is to change the moles to grams using its Ar and Mr.

 

Ar Values :  H = 1     ,   0 = 16

Mr Values :  2H2 = 4    ,   02 = 32  ,   2H20 = 4 + 32 =  36

 

 

4 grams of Hydrogen reacts with 32 grams of oxygen to give 36 grams of water 

 

The mass of Hydrogen + Oxygen at the start = 36 g.

The mass of water at the end = 36g.

As you can see, the total mass does not change during the chemical reaction. 

 

 

 

Calculating Masses from Equations

 

Here is an example:

Magnesium reacts completely with oxygen gas to form magnesium oxide. What mass of oxygen is needed for 24g of magnesium, and what is the mass of magnesium oxide is obtained?

 

Firstly, create an equation and make it balanced.

 

2Mg + O2 ----> 2MgO

 

Then, find its Ar and Mr.

 

Ar Value = Mg: 24   , O: 16

Mr Value = O: 16 x 2= 32    , MgO: 24 + 16 = 40

 

Using the Ar and Mr, change the moles to grams.

 

2Mg = 24 x 2 = 48g , O2 = 32g   , 2MgO = 40 x 2 = 80g

 

But, now you only start with 24g of magnesium so the actual masses are:

 

48/2 + 32g/ 2 ----> 80/2

 

24g + 16g ----> 40g

 

Answer : So, 24g of magnesium needs 16g of oxygen and gives 40g of magnesium oxide,

 

Working out equations, from masses

 

Example:

Iron reacts with a solution of copper(II) sulfate (CuSO4) to give copper and iron sulfate. The formula of iron sulfate is either FeSO4 or Fe2(SO4)3. 1.4g of iron gives 1.6g of copper. Write a correct equation.

 

 

Firstly, get the Ar of the products used.

Ar: Fe = 56     , Cu = 64

 

 

Then, change the masses to moles of atoms;

 

 

1.4/56 moles of iron gives 1.6/64 moles of copper,

0.025 moles of iron gives 0.025 moles of copper.

 

 

The moles are in a 1:1 ratio which means that 1 mole of iron gives 1 mole of copper.

 

 

Equation: Fe + CuSO4 ----> Cu + FeSO4

 

 

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