Relative formula masses and molar volumes of gases (Section 1d)

Relative formula masses and molar volumes of gases

1.16 calculate relative formula masses (Mr) from relative atomic masses (Ar)

The relative formula mass of a compound is the relative atomic masses of the atoms it contains added together. (number of this element within molecule x relative atomic mass of element) + (number of this element within molecule x relative atomic mass of element.) 

For example, to find the relative formula mass (Mr) of MgCl2 (magnesium chloride);

There is one element of Mg within molecule, and the atomic mass = 24.

There are 2 elements of Cl within molecule, and atomic mass = 35.5

So, relative formula mass = (1 x 24) + (2 x 35.5)

                                      = 24 + 71

                                      = 95.

1.17 understand the use of the term mole to represent the amount of substance 

The mole is the amount of substance that contains 6 x 10²³ of that substance. When you have exactly that number of atoms or molecules, they weigh the same number of grams as the relative atomic mass of that element, or relative formula mass of that compound. So, as carbon has an Ar of 12, one mole of carbon will weigh exactly 12g. Also, as carbon dioxide has an Mr of 44, one mole of carbon dioxide will weigh exactly 44g. 

1.18 understand the term mole as the Avogadro number of particles (atoms, molecules, formulae, ions or electrons) in a substance 

The number 6 x 10²³  is known as Avogadro's number and means that a mole refers to the Avogadro's number of particles in a substance. These particles can be atoms, molecules, formulae, ions or electrons.

1.19 carry out mole calculations using relative atomic mass (Ar) and relative 

formula mass (Mr) 

The formula for finding the number of moles in a given mass is as follows; 

Number of moles = Mass in g of element or compound/Mr of element or compound 

Example, How many moles are there in 66g of carbon dioxide?
Mr of carbon dioxide (CO2) = (1 x 12) + (16 x 2)
                                         = 12 + 32 
                                          = 44
Number of moles = mass in grams/Mr 
                          = 66/44
                          = 1.5 moles.

So, there are 1.5 moles in 66g of carbon dioxide.

1.20 understand the term molar volume of a gas and use its values 
(24 dm3 and 24,000 cm3) at room temperature and pressure (rtp) in 

calculations. 

The molar volume of a gas is the volume that one mole of gas fills. One mole of any gas will always occupy 24dm3 (24,000cm3) at room temperature and pressure. RTP = 25 degrees celsius, and 1 atmosphere.) 

Therefore, you can use the following formula to convert the number of moles or mass of any gas to a volume;

Volume (dm3) = moles of gas x 24