C4 Chemical Calculations

Question 1

What is the relative atomic mass of an element?

Answer 1

Atom mass is tiny
So relative mass of different atoms is used
The standard atom is carbon -12, 12/6C
It has 6 protons and 6 neutrons
You compare the mass of other elements against it

Question 2

How are isotopes treated?

Answer 2

The average number of isotopes is used so chlorine is 35.5

Question 3

How do you work out relative formula mass?

Answer 3

This is also called the relative molecular mass depending on whether you are dealing with ionic substances
To work out relative molecular mass add up mass numbers of the elements in question
Use the relative atomic mass to calculate the relative molecular mass

Question 4

What is atomic number?

Answer 4

Number of protons which is the same as the number of electrons

Question 5

What is mass number?

Answer 5

Number of protons add the number of neutrons

Question 6

What is a mole of a substance?

Answer 6

One mole is Avogadro’s number 6x10(23) of particles
It is the relative atomic mass measured in grams
Moles = mass/Mr
Eg H2O has Mr of 18 because (1x2)+16
The mole allows us to work out how much of a substance produced or required for any reaction

Question 7

Example:

What mass of calcium oxide can be made from 200g calcium carbonate?

Answer 7

Step 1
Balanced equation - CaCO3-> CaO + CO2
Work out number of moles - moles = mass/Mr - 200/100
Therefore this is 2 moles of calcium carbonate

Step 2 ratio
Balance numbers are all 1 in this equation
But we start with 2 moles therefore will make 2 moles of calcium oxide

Step 3 rearrange
Mr x moles = mass
Mr of calcium oxide is 56 
56x2=112
So from 200g calcium carbonate we expect 112g of calcium oxide

Question 8

What is the limiting reactant?

Answer 8

The reactant that gets used up first in a reaction is called the limiting reactant
The limiting agent has less moles in the experiment
The excess agent has the more moles

Question 9

What is the equation for concentration?

Answer 9

Concentration mol/dm3= moles/volume (dm3)

Make sure you change volume to dm3

Question 10

Another measure of concentration?

Answer 10

Concentration (g/dm3) = concentration (mol/dm3) x Mr

Question 11

What does concentration also equal?

Answer 11

Amount of solute / volume of solution