Module 2, Chapter 3: Amount of Substance

What is the Amount of Substance measured in?
What does one mole equal?
the amount of substance that contains 6.02 x 10^23
What is Avogadro's constant?
6.02 x 10^23
What does Avogadro's constant equal to?
the number of particles in each mole of carbon - 12
What does the mass of 1 mole of atoms of an element equal to?
the relative atomic mass in grams
Moles (in terms of mass and Mr) =
mass / Mr
Units of molar mass
g/mol or gmol ^-1
Amount of substance =
mass / molar mass
Molecular Formula
the actual number and kinds of atoms in a molecule
two or more atoms held together by covalent bonds
Empirical Formula
the simplest whole number ratio of atoms of each element in a compound
What is the Empirical Formula important for?
it is important for substances that don't exist as molecules
Relative Molecular Mass
the weighted mean mass of an atom of an element, compared with 1/12 of the mass of a molecule with the mass of an atom of carbon 12
Relative Formula Mass
The weighted mean mass of the formula unit of a compound, compared with one-twelfth of the mass of an atom of carbon-12
water molecules are part of their crystalline structure
What are the water molecules that are part of a crystalline structure called?
'The Water of Crystaliisaztion'
How is the Water of Crystallization is shown?
with a large dot
An experiment to determine the 'Water of Crystallisation' in Hydrated Salts
1. weight an empty crucible

2. add hydrated salt

3. weigh crucible and contents

4. heat contents strongly until constant mass

5. weigh crucible + anhydrous salt
Calculation of Formulae using An experiment to determine the 'Water of Crystallisation' in Hydrated Salts
1. mass of anhydrous salt

2. mass of water

3. divide by smallest number
Assumptions we make in An experiment to determine the 'Water of Crystallisation' in Hydrated Salts
- All the Water Evaporates

- If heated too strongly: the salt decomposes
In an experiment to determine the 'Water of Crystallisation' in Hydrated Salts, what if all the water doesn't evaporate?
- final mass is too high

- calculated mass of water too low

- formula Would have a lower number of 'Water of Crystallization'
In an experiment to determine the 'Water of Crystallisation' in Hydrated Salts, what if i's heated too strongly
- the salt decomposes

- final mass too low

- difference would be too high

- formula would have a high number of 'Water of Crystalisation'
What does Volume measure?
liquids or gases
1l =
1 dm3 = 1000cm3 = 1000ml
Standard Solution
is a solution of known concentration
How are Standard Solutions prepared?
by dissolving an exact mass of the solute in a solvent and making up the solution to an exact volume
At the same temperature and pressure.......
equal volumes of different gases contain the same number molecules
The Molar Gas Volume
the volume per mole of gas molecules at a stated temperature and pressure
What does the Volume of Gas depend on?
pressure and temperature, but many experiments are carried out a room temperature and pressure
Amount (mol)
volume / molar gas volume
you will have to come across the following assumptions for the molecules making up an ideal gas:
- Random Motions

- Elastic Collisions

- Negligible Size

- No intermolecular forces
pV =
the ratio in a balance
Chemists use balanced equations to find:
- the quantities of reactions required to prepare a required quantity of a product

- the quantities of products that should be formed from certain quantities of reactants
1. Work out the amount of moles of whatever you can

2. Use the equation to work out the amount in moles of the unknown chemical

3. Work out the unknown information required
Theoretical Yield
the maximum possible amount of product
Why is the Theoretical Yield difficult to achieve?
- the reaction may not have gone to completion

- other reactions (side reactions) may have taken place alongside the main reaction

- purification of the product may result in loss of some product
% Yield =
actual yield/theoretical yield x 100
Volume =
Concentration =