By the Tadrees team
Chemistry combines facts you must remember with calculations you must practise. In MCQ exams you need both, and you need them to be quick. This guide explains how to prepare the three areas that cause students the most trouble: chemical formulas, equations and the mole concept.
Writing a formula correctly starts with the charge or valency of each part. Learn the common ions once: for example sodium Na+, calcium Ca2+, aluminium Al3+, chloride Cl−, oxide O2−, sulphate SO42−, nitrate NO3−. Then build formulas by balancing charges: calcium chloride needs two Cl− for each Ca2+, giving CaCl2. Once you understand this, you can write hundreds of formulas without memorising each.
Balancing is a counting exercise. Write the unbalanced equation, then count atoms of each element on both sides. Balance elements that appear in only one substance on each side first, leave hydrogen and oxygen until last, and adjust coefficients, never the small numbers inside formulas. At the end, count every element again. Practise with a variety of reaction types, including combustion, neutralisation, displacement and precipitation, until the routine is automatic.
Many MCQs ask for the product of a reaction. Learning the patterns helps a lot: metal + acid gives salt + hydrogen, metal + oxygen gives metal oxide, acid + base gives salt + water, carbonate + acid gives salt + water + carbon dioxide. Add the common observations as well, such as effervescence, colour changes or a precipitate forming.
Almost every stoichiometry question uses a small set of relationships:
Keep these on your fact sheet and practise moving between them. Most numerical MCQs are just two or three of these steps chained together.
Following the same four steps every time makes the problem mechanical and cuts down on mistakes. Always check that your answer has the right unit and a sensible size.
First practise recall of formulas and reactions with short factual questions. Then move to balancing and product prediction, and finally to mole calculations. Reading the explanation of every worked question shows you the steps, which is how you build speed. Tadrees groups chemistry questions by topic so you can climb these layers one chapter at a time.
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