If you are unsure of the answer to a multiple choice question, don’t spend too long on it. Put a star by it and return to it later.
Within a single multiple choice question, use a pencil to cross out the WORD/PHRASE of the statements which are clearly incorrect, then choose between those left.
In a multiple choice question don’t be swayed by one of the choices just because it has got a longer (or shorter) statement than the others.
Don’t make any assumptions about the order of responses – just because there have been two answers ‘D’ in sequence, it does not mean that the next answer cannot be ‘D’.
Take care to read the whole question word by word. For example, in the question ‘what is the ratio of the volumes of 2g of H2 and 16g of methane, CH4, at r.t.p?’ Many students will focus on the numbers and ignore the word ‘volume’. Just a quick look at the figures gives the incorrect answer 1:8 (using the molar gas volumes gives 1:1 as the correct choice)
Common Mistakes for MCQ
When given a choice of picking out a noble gas from a group of electronic structures, don’t jump to the conclusion that noble gases always have 8 electrons in their outer shell. Remember that helium has 2!
When given a choice about electrical conductivity of ionic structures remember that the conduction is due to IONS moving (not electrons). The ions can only move when the ionic compound is molten or when dissolved in water.
When given choices of why alloys are hard, it’s not the mass of the atoms which is important but their size. Remember that metals have layers which slip over each other. A different sized atom will distort the layers and stop them slipping over each other. This makes the alloy harder than the pure metals.
When given choices about the rate of diffusion of gases, remember that the rate of diffusion depends on the mass of the molecules. Heavier molecules (lower relative molecular mass) move and diffuse slower than lighter molecules. If you are unsure which molecule is heavier, use your Periodic Table to calculate the relative molecular masses.
If you are given a choice of tap water and several other substances as examples from which to select a pure compound, it’s not going to be tap water. It is a common error to think that tap water is pure. It contains compounds dissolved from the rocks or carried in the rain as well as the chemicals put in to purify it. It’s a mixture. (Don’t be fooled by the adverts of the mineral water companies which say ‘pure mineral water’!)
If you are given choices of electronic structures of atoms to select to make a compound of type XY2, first check the type of compound that the examiner wants e.g. ionic or covalent. If it is ionic, then you can choose an atom with one or two electrons in its outer shell and combine it with a non-metal atom. If it is covalent look for the structures of two non-metal atoms i.e. those with 4 to 7 electrons in their outer shell. Remember that the number of electrons in the outer shell is equal to the group number.
Remember that the valencies of the elements in Groups V to VII are found by taking the group number away from 8. For example, the valency of oxygen in an oxide is 8-6 = 2. (Oxygen is in Group VI)
Hope you find these useful! Will share more tips for structured questions in the future.
A Chem-Addict passionate about teaching and learning Chemistry.