Draw a simple model of the atom, label all of the parts with their charge and mass.

Atoms are very small, having a radius of

about 0.1 nm (1 x 10-10 m).

The radius of a nucleus is less than 1/10 000 of that of the atom (about 1 x 10-14 m).

Useful calculations:

The mean (average):

Add up all the values you are given and divide by the answer by the number of values.

The range:

This is the highest value given minus the lowest value given

% uncertainty = range

mean

Uncertainty of equipment =

the smallest reading it can take

2

Amounts of substances in equations (HT ONLY)

Using balanced symbol equations the masses of reactants and products can be calculated.

For the reaction:

The equation also shows us that 2 moles of magnesium will react with 1 mole of oxygen to produce 2 moles of magnesium oxide.

Mg = 24 O =16

So...

48g + 32g —> 80g

To calculate different masses the equation is needed:

molar mass of substance A = molar mass of substance B

mass of A mass of B

Worked example – calculate the mass of MgO made from 6.0g of Mg.

Rearrange the equation to:

Mass of B = mass A x molar mass of B

Molar mass of A

Substitute in numbers

Mass of MgO = 6.0 x 80

48

Calculate = 10g Don’t forget the units

What is relative atomic mass (Ar) and how can you find it?

The Relative Atomic Mass (RAM or Ar) is calcu-lated in comparison to CARBON 12. It is the sum of the protons and neutrons in the nucleus.

What is relative formula mass (Mr) ?

The Relative Molecular Mass (RMM or Mr) is calculated using the RAM/Ar of the atoms making up the molecule. Mr of CaCO3 = Ar of Ca + Ar of C + 3xAr of O = 40 + 12 + 3x16 = 48 = 100 Remember – in a balanced equation, the sum of the Mr of the reactants equals the sum of the Mr of the products – this shows conservation of mass.

Moles and reacting mass (HT ONLY)

Avogadro's number 6.02 x 1023 atoms is the num-ber of atoms in the relative atomic mass of an at-om.

So, a 24 g piece of magnesium contains

6.02 x 1023 atoms.

This also refers to one mole of a substance.

Calculating molar mass

-Unit is g/mol or gmol-1.

-The mass of one mole of a substance is calculat-ed by adding up the relative atomic masses of the atoms in the formula.

Eg for H2O

H + H + O = H2O

1 + 1 + 16 = 18g

One mole of water = 18g/mol

Eg for formula containing brackets, these must be considered in the calculation:

Mg(NO3)2 atoms in the brackets must be multi-plied by 2

Mg = 24, N = 14, O = 16

So —> 24 + (2x14) + (2x(3x16) = 148 g/mol.

How can we work out the number of moles in a sub-stance? (HT ONLY)

Mass equals Mister Moles (mass = Mr x moles)

(HT ONLY) Mole ratios!

2H2O2—> O2 + 2H2O

How many moles of H2O2 are needed to make …

1 mole of O2?

If 2 moles of H2O2 make 1 mole of O2 then halving the amount of H2O2 (from 2 moles to 1 mole) will half the amount of O2 made to half a mole.

3 moles of O2?

3 moles of O2 can only be made if there is three times the amount of H2O2 needed to make 1 mole of it. 3 x 2 moles = 6 moles.

123.5—79.5 = 44

Chemistry Unit 3 knowledge organiser—Chemical quantities and calculations

Dissolving keywords:

Solute: the chemical that is dissolved (often a solid like salt)

Solvent: The liquid that chemicals are dis-solved in

Solution: a mixture of a solvent and a solute

Concentration: the mass of solute that is dis-solved in a certain volume of solvent.

Soluble—will dissolve

Insoluble—won’t dissolve

Knowledge quiz 1—everyone

1.Which sub atomic particles have a mass of 1?

2.Which number on your periodic table shows the mass of an element?

3.What is the mass of CH4?

4.What is the law of conservation of mass?

5.10g of Mg burns in 10g of O2. What mass of MgO is made?

Knowledge quiz 2—everyone

1.What is a limiting reactant?

2.What is a reactant ‘in excess’?

3.Why do reactions often look like the prod-ucts weigh less than the reactants?

4.Why do metals weigh more after heating in oxygen?

5.How do you calculate mass change?

Knowledge quiz 3 everyone

1.How is the mean/average calculated?

2.How is the range calculated

3.How do you calculate % uncertainty?

4.How do you find the uncertainty of a piece of equipment?

5.What is the uncertainty of a balance that can measure 0.01g

Knowledge quiz 4 Higher only

1.How many particles are in 1 mole?

2.How can you work out the number of moles?

3.What is the mass of half a mole of MgO?

4.Look at the equation: 2Mg + O2 à 2MgO. How many moles of MgO will be made if 2 moles of O2 are used?

5.What are the units for relative formula mass?

Knowledge quiz 5 everyone

1.What causes an increase in the concentra-tion of a solution?

2.What formula do you need to work out con-centration?

3.What units do chemists use for volume?

4.How do we convert cm3 into these units?

5.What is the concentration of 100cm3 of wa-ter with 5g of salt dissolved in it?

What are the potential sources of error in the experiments above?

Misreading scales when weighing out reac-tants

Misreading scales when weighing products

Incorrect measurements of solutions us-ing a measuring cylinder

How could accuracy be improved?

-Use equipment that allows us to measure to more decimal places (higher resolution).

Eg a balance that can measure up to 0.001g.

What calculation is needed to work out the concentration of a solution?

CONCENTRATION – the amount of a chemical dissolved in a certain volume of a solution.

It is calculated using:

Concentration = mass of solute

volume

The units for volume is dm3 this is equal to 1000cm3 (to convert cm3 to dm3 divide by 1000).

Worked example:

A solution has a concentration of 4.2g/dm3. Cal-culate the mass of solute dissolved in 250 cm3 of solution.

1. Use the equation & Substitute in known values.

Concentration = mass of solute

Volume

4.2 g/dm3 = ????????

( 250/1000)

2. Rearrange

Mass of solute = 4.2 x (250/1000)

= 1.05 g (Don’t forget the units!)

How can we calculate the % of an element in a compound?

% composition =

Mr of element x 100

Mr of compound

Worked example :

What % of H2SO4 is sulfur?

Mr of S = 32, Mr of H2SO4 = 98

32 x 100

98

=33% (after rounding)

Mass solute*

Concentration

volume

* HIGHER you can also swap mass for moles to work out how many moles are in a solution