What is being taught lesson by lesson:
- Relative Formula Mass – also called Mr and RAM. How to calculate it and link it to isotopes studied last year.
- The conservation of mass. This links to experiments, calculating masses of reactants or products and balancing equations.
- Measurements and uncertainty. While we are doing lots of maths, it is time to address accuracy of the equipment we use and quantify it.
- Moles! The bit of chemistry that can be scary but one you’ve practised it, you’ll love it. It is the key part of all chemical calculations…Thanks Avogadro!
- Calculations using moles such as the reacting masses and involving limiting reactants.
- Solutions. Use moles to calculate concentration of a solution in both gdm-3 and moldm-3. This can then be used in calculations to find unknown concentrations too. (Titrations)
- More practice of the above (practice really does make perfect).
Key Terms for this topic (Tier 3 vocabulary)
Relative atomic mass – isotope – relative formula mass – mole – Avogadro’s constant – conservation of mass – reacting masses – limiting reactants – concentration – neutralisation – dilution.
Are you ready for your assessment in this topic? Try out this simple quiz.
What everyone needs to know:
The law of “conservation of mass” states that the total mass at the start of a reaction is the same as the mass after – not atoms are gained or lost.
By adding the Ar (relative atomic mass) of atoms in a compound, you can work out the Mr (relative formula mass). You can calculate the percentage mass of an element in a compound and use this to calculate what mass of an element there is in a known mass of a substance.
When one or more of the products in a gas, the reaction may appear to lose mass, it doesn’t, some of the atoms simply escape as a gas but if they were trapped, conservation of mass prevails. Likewise, burning metals appear to get heavier, this is simply gaseous oxygen combining with the metal to form a metal oxide. The mass does not “appear”, it was always there in the atmosphere.
Depending on the measurements you take and the equipment you use, there is a level of uncertainty. You need to be able to describe the uncertainty and suggest ways of improving it.
When a solid is dissolved in water, you need to be able to calculate its concentration in g/dm3.
Extra topics needed for the Higher papers:
You need to be able to find out the number of moles (mol) in a substance using its mass and relative formula mass. This is linked to the Avogadro constant of 6.02 x 1023. Link this to equations, e.g. 1 mole of sulfuric acid reacts with 2 moles of sodium hydroxide.
Using balanced symbol equations, calculate the mass of a product or reactant using the mass of one that is known (reacting masses). Need to be able to apply ratios here too.
The limiting reactant is the one to “run out” first. You need to identify which one this is (other reactants are in excess) and use it in your calculations.
As well as the solutions noted in the foundation topics above, you need to be able to measure and calculate concentration in mol/dm3.