Chemistry 5070/12 — October/November 2020

Walkthrough

In a titration, the volume of liquid delivered from the burette is found by subtracting the initial reading from the final reading. Burette scales are unique because the numbers increase downwards (0 at the top, higher numbers at the bottom). Readings must always be taken at the bottom of the liquid meniscus.

From the diagrams:

• The start of titration reading is $22.7\text{ cm}^3$.
• The end-point reading is $27.6\text{ cm}^3$.

Subtracting the start value from the end value:

$$27.6\text{ cm}^3 - 22.7\text{ cm}^3 = 4.9\text{ cm}^3$$

This matches option B.

Key Takeaways

• Burette scales increase downwards, so the end reading is always larger than the start reading.
• Always read the volume at the bottom of the meniscus.
• Volume delivered = final reading - initial reading.

Common Mistakes

• Subtracting the start reading from the end reading in the wrong order (e.g., $22.7 - 27.6$).
• Reading the top of the meniscus instead of the bottom.
• Misreading the scale direction (thinking 22.7 is above 23 when it is below).
• Forgetting that the unit is $\text{cm}^3$ and not $\text{dm}^3$.

Things to Be Careful About

• Burette readings are always to 1 decimal place (e.g., $22.70$ is acceptable but $22.7$ is standard for this level).
• Ensure the final answer is positive; if you get a negative number, you subtracted in the wrong order.
• The scale on a burette is inverted compared to a measuring cylinder.