A level chemistry maths skills tips
Some last minute tips to get you some vital marks when doing calculation questions. Some might sound simple, but I know that people make have made all of these mistakes!
1) Make sure you know the difference between significant figures and decimal places.
2) Remember that the relative masses on your periodic table go to 1 decimal place.
3) Check that the numbers you put in your calculator are the same as the numbers you were given.
4) Check that your answer is to the number of significant figures that the question wants. It may as for a set number of significant figures. If it tells you to give your answer to an appropriate number of significant figures or does not say anything, then give your answer to the same number of significant figures as the number(s) in the question with the least number of significant figures.
5) When doing mole ratios in an equation, make sure you know whether to multiply or divide to get the number of moles you need.
For example, if you are given the number of moles of Manganate (VII) ions for the equation below, you need to multiply the value by 5 to get the number of moles of iron ions.
6) Manganate (VII) ions take 5 electrons, dichromate ions take 6 electrons.
7) When doing equilibrium equations and redox equilibria equations, make sure you have the sign of the enthalpy/electrode potential number the right way round for the equation.
8) In questions where you add an acid and an alkali together to work out the new pH, remember that if they are both solutions that you need to add the volumes together. If you add a solid alkali (it is given in grams rather than ml), then just use the volume of the acid.
9) Make sure you know the units for each number you are given.
10) Make sure you convert the units you need so that all the units that measure the same thing are the same.
11) Expand brackets as late as possible.
12) Try to find values that are the same. For example, in a time of Flight question, the question stated that the kinetic energy for two different ions was the same. This means that you had to do 1/2mv2 for each one and then make them equal each other to work out the answer.
13) Calculate things so that you don't have to switch the + and - signs around.
For enthalpy change, it is products - reactants. Don't keep trying other stuff and wondering whether you need to change the signs of the products and reactants.
For a Born Haber cycle, start with the elements in their standard state and draw a circle going all the way round to the ionic compound. This circle will go in the same direction as all the arrows, so you don't need to change any signs. The sum of all those numbers is equal to the enthalpy of formation, which is the arrow that goes down. All you then have to do is rearrange the equation to make the unknown enthalpy the subject.