GSCE Chemistry C5 OCR

Molar Mass

What is a mole?

It is the amount of substance in a chemical reaction and is measured in moles. 

The mass of one mole (the molar mas) of any substance is the relative formula mass (Mr), in grams (g)

For Example:

What is the Molar mass of sodium hydrioxide? (NaOH)

Na     1 x 23   = 23

O       1 x 16   = 16

H        1 x 1    =   1

Relative forumla mass = 23 + 16 + 1 = 40

So the relative formula mass of NaOH is 40 and so the mass of 1 mole of NaOH is 40g.

During a chemical reaction, no mass is lost or gained: it is conserved.

For example, when 100g of calcium carbonate decomposes, it produces 56g of calcium oxide and 44g of carbon dioxde:

Calcium Carbonate      -------------> Calcium oxide       +         Carbon Dioide 

     100g                                                56g                                44g

    100g                      ------------->    100g

However, the mass measured at the end of a reaction might be.....

• greater if a gas has been gained from the air or
• Smaller if water vapour or a gas had been allowed to escape.

An example question to try:

When 50g calcium carbonate is heated in a thermal decomposition reaction, 28g of calcium oxide is made. What mass of carbon dioxide is lost?

Answer on next page

So 

50g -------------> 28g + mass of carbon dioxide 

Mass of Carbon dioxide = 50g - 28g = 22g 

Mass of Carbon dioxide = 22g 

Reacting Ratios 

If the reacting masses