Chemistry Multiple Choice Questions (Internal Exam)
Topics to work on
- Relative atomic mass and isotope calculation
- Calculating weighted averages using isotopic mass and percentage abundance
- Applying significant figure rules strictly to calculated atomic masses
- Acid-base theory and dissociation equations
- Differentiating strong acids from weak acids
Multiple choice questions
| relative mass of isotope | abundance / % |
|---|---|
| 238 | 66 |
| 244 | 34 |
- A sample of element Q has two isotopes. Their relative masses and abundances are shown. What is the relative atomic mass of this sample of Q to three significant figures?
- Correct: A (240)
- Why it’s CORRECT: In order to calculate the relative atomic mass of the sample, you calculate the weighted average of the isotope masses based on their percentage abundances: (mass₁ x abundance₁) + (mass₂ x abundance₂) / total abundance.
- Multiply each mass by its abundance. The first isotope would be 15,708 (238 x 66) and the second isotope would be 8,296 (244 x 34).
- Sum the values and divide by 100%. The total sum would be 24,004 (15,708 + 8,296). After dividing by the total abundance, 24,004 / 100, you would get 240.04.
- Round to three significant figures (in instructions). 240.04 rounded to 3 significant figures gives 240. Therefore, the correct choice in the question is A.
- Why it’s CORRECT: In order to calculate the relative atomic mass of the sample, you calculate the weighted average of the isotope masses based on their percentage abundances: (mass₁ x abundance₁) + (mass₂ x abundance₂) / total abundance.
- Chose B.
- Why it’s WRONG: Most likely a guess or wrong calculation.
- Correct: A (240)
| hydrochloric acid | ethanoic acid | |
|---|---|---|
| A | HCl(aq) → H⁺(aq) + Cl⁻(aq) | CH₃COOH(aq) ⇌ H⁺(aq) + CH₃COO⁻(aq) |
| B | HCl(aq) ⇌ H⁺(aq) + Cl⁻(aq) | CH₃COOH(aq) ⇌ H⁺(aq) + CH₃COO⁻(aq) |
| C | HCl(aq) → H⁺(aq) + Cl⁻(aq) | CH₃COOH(aq) → H⁺(aq) + CH₃COO⁻(aq) |
| D | HCl(aq) ⇌ H⁺(aq) + Cl⁻(aq) | CH₃COOH(aq) → H⁺(aq) + CH₃COO⁻(aq) |
- Which row shows the equations for the dissociation of hydrochloric acid and of ethanoic acid in aqueous solution?
- Correct: A
- Why it’s CORRECT:
- Hydrochloric acid is a strong acid. Strong acids dissociate completely in aqueous solution, and complete dissociation is represented by a single forward arrow.
- Ethanoic acid is a weak acid. Weak acids only partially dissociate in aqueous solution, setting up a dynamic equilibrium. Partial/reversible dissociation is represented by a reversible reaction arrow.
- Why it’s CORRECT:
- Chose C.
- Why it’s WRONG: In C, the equation for the dissociation of ethanoic acid has a single forward arrow. This is wrong because ethanoic acid partially dissociates.
- Correct: A