Moles and Empirical Formula - 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)
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.
Moles and Empirical Formula - Conservation of Mass
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
Answer to Practice Question
50g -------------> 28g + mass of carbon dioxide
Mass of Carbon dioxide = 50g - 28g = 22g
Mass of Carbon dioxide = 22g
Moles and Empirical Formula - Reacting Ratios
If the reacting masses