Moles and Compounds

The idea of the mole has not been only used with elements and atoms but also used with compounds.

1 mole of water H₂O (left) and 1 mole of ethanol C₂H₅OH (right) in measuring cylinders

If you asked what will be the mass of 1 mole of water (H₂O) molecules? (where, Ar for H = 1 and O = 16)

Let’s find the solution.

From the formula of water, H₂O, we can see that 1 mole of water molecules contains 2 moles of hydrogen (H) atoms and 1 mole of oxygen (O) atoms.So we get the mass of 1 mole of water molecules is = (2 × 1) + (1 × 16) = 18 g

The mass of 1 mole of any compound is called its molar mass. One can write the molar mass of a compound without any units. Then it is called the relative formula mass, often called the relative molecular mass (M_r ).

So the relative formula mass (RFM) of water is = 18.

Let’s learn and understand more about moles and compounds! 

Example 1

What will be the mass of  a) one mole and b) the relative formula mass (RFM) of ethanol, C₂H₅OH?  (A_r For H = 1, C = 12 and O = 16)

a) 1 mole of C₂H₅OH contains 2 moles of carbon atoms, 6 moles of hydrogen atoms and 1 mole of oxygen atoms. So, the mass of one mole of ethanol is

= (2 × 12) + (6 × 1) + (1 × 16)

= 46 g

b) The RFM of ethanol is 46.

Related- Calculating Moles: Moles and Elements

Example 2

What will be the mass of a) 1 mole and b) the RFM of nitrogen gas, N₂? (A_r for N = 14)

a) We know Nitrogen (N₂) is a diatomic gas. Each nitrogen molecule contains two atoms of nitrogen.

So, the mass of 1 mole of nitrogen is

= N₂ = 2 × 14

 = 28 g

b) The RFM of N₂ is 28.

The mass of any compound found in any number of moles can be calculated using the following formula

 

Example 3

Calculate the mass of a) 3 moles and b) 0.4 moles of carbon dioxide gas, CO₂ . (A_r for C = 12 and O = 16)

1 mole of CO₂ contains 1 mole of carbon atoms and 2 moles of oxygen atoms.

So the mass of 1 mole of CO₂ is

 = (1 × 12) + (2 × 16)

= 44 g

 

a) The mass of 3 moles of CO₂ is

= number of moles × mass of 1 mole of CO₂

= 3 × 44

= 132 g

Similarly,

b) the mass of 0.4 mole of CO₂ is = 0.4 × 44 = 17.6 g

If you know the mass of a compound then you can calculate the number of moles of the compound using the following relationship

 

To know- Electrophoresis and DNA Fingerprint 

Example 4

We can calculate the number of moles of magnesium oxide, MgO in  a) 80 g and b) 10 g of the compound. (A_r for O = 16 and Mg = 24)

1 mole of MgO contains 1 mole of magnesium atoms and 1 mole of oxygen atoms.

So, the mass of 1 mole of MgO

= (1 × 24) + (1 × 16)

= 40 g

a)The number of moles of MgO in 80 g

= 80 g ÷ 40g = 2

Similarly

b) the number of moles of MgO in 10 g is

= 10 g ÷ 40g

= 0.25 mole

Source- Cambridge IGCSE Chemistry 3rd Edition