Five common mistakes in CSCA Chemistry and how to avoid them

Chemistry questions are often short, but small slips cost marks. Here are five mistakes we see again and again in practice.

1. Dividing by the wrong mass

Mass fraction is solute ÷ solution, not solute ÷ water. Add the masses first.

2. Forgetting the 2 in sulfuric acid

One mole of $\mathrm{H_2SO_4}$ gives two moles of $\mathrm{H^+}$. This changes the pH.

3. Mixing up collection methods

A gas that dissolves in water cannot be collected over water. Compare the relative molecular mass with 29 to decide between upward and downward displacement of air.

4. Ion pairs that cannot coexist

$\mathrm{H^+}$ with $\mathrm{OH^-}$, $\mathrm{Ag^+}$ with $\mathrm{Cl^-}$, $\mathrm{Ba^{2+}}$ with $\mathrm{SO_4^{2-}}$ — learn the common pairs as a list.

5. Unsafe lab answers that "sound right"

Never add water to concentrated sulfuric acid. Pour the acid slowly into water while stirring.

Build your own list

Every time you get a question wrong in chemistry practice, choose the reason. After two weeks your mistake book will show which reason appears most — that is the habit to fix first.

中文摘要

化学常见五错：质量分数分母应是溶液质量；硫酸一摩尔给两摩尔氢离子；会溶于水的气体不能用排水法；记住不能共存的离子对；稀释浓硫酸要把酸倒入水中。用错题本统计自己最常见的错因。