The Mole & the Avogadro Constant (Edexcel A Level Chemistry): Revision Note

1. What is the molar mass of water?

2. How many moles are there in 100 g of water?

3. How many water molecules are there in 100 g of water?

Answers

1. Molar mass of water, H₂O = (2 x 1.0) + 16.0 = 18.0 g mol^-1 

2. Moles $=\frac{mass}{molarmass}=\frac{100}{18.0}=$5.56 moles (to 3 s.f.)

3. Number of molecules = number of moles x Avogadro's constant = 5.56 x (6.02 x 10²³) = 3.35 x 10²⁴ molecules

What is the mass of the following:

1. Five hundred million atoms of platinum

2. (1.31 x 10²²) molecules of ethanol

Answer 1:

Number of moles $=\frac{\mathrm{number}\mathrm{of}\mathrm{particles}}{\mathrm{Avogadro}\text{'}\mathrm{s}\mathrm{constant},{\mathrm{N}}_{\mathrm{A}}}=\frac{500\times {10}^6}{6.02\times {10}^{23}}=$8.31 x10^-16 moles

Mass = moles x molar mass = (8.31 x10^-16) x 195.1 = 1.62 x 10^-13 g (lots of atoms, a tiny mass)

Answer 2:

Molar mass of ethanol, C₂H₅OH = (2 x 12.0) + (5 x 1.0) + 16.0 + 1.0 = 46.0 g mol^-1 

Number of moles $=\frac{\mathrm{number}\mathrm{of}\mathrm{particles}}{\mathrm{Avogadro}\text{'}\mathrm{s}\mathrm{constant},{\mathrm{N}}_{\mathrm{A}}}=\frac{1.31\times {10}^{22}}{6.02\times {10}^{23}}=$0.0218 moles

Mass = moles x molar mass = 0.0218 x 46.0 = 1.00 g (an exceptionally large number of molecules in just 1 g)

• When you are completing calculations using Avogadro's constant, you may end up with answers that seem very large or very small - don't automatically assume that they must be wrong

• Remember, Avogadro's constant is a VERY large number:

  • 6.02 x 10²³ or 602 000 000 000 000 000 000 000

• So when you multiply or divide by Avogadro's constant, your answers will, naturally, become very large or very small