Chemistry 5070/11 — May/June 2021

Walkthrough

The question asks for the correct apparatus to measure exactly $28.2\text{ cm}^3$ of aqueous sodium bromide. The value $28.2\text{ cm}^3$ has one decimal place, meaning the apparatus must be able to read to at least $0.1\text{ cm}^3$.

• Apparatus A (beaker): Beakers have graduation marks but are only used for rough estimates of volume, typically accurate to $\pm 5\text{ cm}^3$. They cannot measure $28.2\text{ cm}^3$ accurately.
• Apparatus B (burette): A burette has graduations along its length and a tap at the bottom, allowing the user to deliver a variable volume of liquid very accurately, typically to $0.05\text{ cm}^3$ or $0.1\text{ cm}^3$. This matches the required precision for $28.2\text{ cm}^3$.
• Apparatus C (volumetric pipette): This apparatus is designed to measure one specific, fixed volume (such as $25.0\text{ cm}^3$ or $50.0\text{ cm}^3$) very accurately. It cannot be used to measure a variable volume like $28.2\text{ cm}^3$.
• Apparatus D (measuring cylinder): While more accurate than a beaker, a standard measuring cylinder is usually only accurate to $1\text{ cm}^3$ or $0.5\text{ cm}^3$. It would not allow the student to measure $28.2\text{ cm}^3$ to the required precision.

Therefore, the burette (B) is the correct choice.

Key Takeaways

• Different apparatus have different levels of precision. Beakers are for rough measurements, measuring cylinders for moderate precision, and burettes or pipettes for high precision.
• Volumetric pipettes are for fixed volumes; burettes are for variable volumes measured to high precision.
• Always check the required decimal places in the volume to determine the necessary precision of the apparatus.

Common Mistakes

• Choosing a measuring cylinder (D) because it can measure $28.2\text{ cm}^3$, but forgetting that its precision is usually only to $1\text{ cm}^3$, not $0.1\text{ cm}^3$.
• Choosing a volumetric pipette (C) because it is precise, but forgetting that it only measures one fixed volume (e.g., $25.0\text{ cm}^3$), not $28.2\text{ cm}^3$.
• Choosing a beaker (A) and ignoring the precision requirement of the measurement.

Things to Be Careful About

• Pay attention to the number of decimal places in the volume. $28.2\text{ cm}^3$ requires an apparatus that reads to at least $0.1\text{ cm}^3$.
• Distinguish between measuring a fixed volume (volumetric pipette) and a variable volume (burette).
• Beakers and conical flasks are not considered measuring apparatus for accurate volumes in Cambridge O Level Chemistry.