Topic 5 - Formulae, Equations and Amounts of Substances - Revision Notes 2015

A Cook 2016
Chemistry Spec Led Revision
Chapter 5 - Formulae, Equations and Amounts of Substance

1. Know that the mole (mol) is the unit for amount of a substance

The Mole:

A mole is the amount of substance that contains the same amount of atoms as 12g of of the 12C
isotope.

2. Be able to use the Avogadro constant, L, (6.02 × 1023 mol-1) in calculations

Avogadro Constant:

Avogadro's constant is 6.02 x 1023, which allows us to scale up small numbers of atoms into grams
- which are easier to visualise.

Calculations using Avogadro constant:

1) Calculate the number of particles in a given mass of a substance
2) Calculate the mass of a given number of particles of a substance

Example 1 -

How many H2O molecules are there in 1.25g of water?

Equation: Moles = Mass / Mole Mass

Moles = 1..0 = 0.0694 mol

0.0694 x (6.0 x 1023) = 4.18 x 1022

Example 2 -

What is the mass of 100 million atoms of gold

Equation: Moles = Mass / Mole Mass

Moles = 100 x .02 x 1023 = 1.66 x 10-16 mol

Mass = 1.66 x 10-16 mol x 197.0 = 3.27 x 10-14 g

3. Know that the molar mass of a substance is the mass per mole of the substance in g mol-1

Molar mass is the mass per mole of any substance (molecular or ionic), and it has the units g mol-1
(grams per mole).

Amount in mol = Mass of substance in g / Molar Mass in g mol-1 or n = m / M

Substance O2 CH4 H 2O NH4NO3

Mass in g 5.26 4.0 100 14.7

Molar Mas in g mol-1 32.0 16.0 18.0 80.0

Amount in mol 0.16 0.25 5.6 0.18

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