OCR A-Level Chemistry AS Breadth in chemistry (01), November 2021: Question 7
1 mark · Easy difficulty · Multiple Choice
Identify the correct procedure regarding the droplet remaining in the tip of a pipette after delivering a solution.
Practise this questionQuestion
Question text
7 After delivering a solution from a pipette, a droplet remains in the tip of the pipette.
How should a student ensure that the pipette delivers the volume of solution stated on the
pipette?
A Fill the pipette just above the graduation line to compensate for the volume of the droplet
that remains in the tip.
B Leave the droplet in the tip.
C Shake the pipette to force out the droplet left in the tip.
D Use a pipette filler to force the droplet out of the tip.
Your answer [1]
Mark scheme
Show the mark scheme
7 B 1 AO2.3
How to answer it
Mastering Volumetric Pipette Technique
What this question tests
This question assesses your practical competency and understanding of how volumetric glassware (specifically a volumetric pipette) is calibrated and used. It tests Application of Science (AO2.3 - applying scientific knowledge to practical procedures).
Handling Droplets in Volumetric Pipettes
✅ Correct Answer
Option B: Leave the droplet in the tip.
Volumetric pipettes are specifically calibrated to account for the small droplet that remains in the tip after delivery. Forcing it out would deliver an excess volume.
💡 Key Knowledge
- Volumetric pipettes deliver a fixed volume (e.g., 25.0 cm³).
- They are calibrated "to deliver" (TD), meaning the manufacturer already factored in the residual droplet remaining in the tip.
- You should never shake or force the final drop out of a pipette.
🧠 Exam Technique
Read practical technique questions carefully. Students often panic when they see liquid left behind in glassware, assuming it causes an error. Trust standard laboratory calibration principles!
❌ Common Errors
- Option C & D: Shaking or forcing out the droplet delivers extra solution, leading to inaccurate titration results (too much reactant added).
- Option A: Overfilling to "compensate" introduces volumetric errors because the pipette is calibrated strictly to the graduation line.
Topics
Module 1: Development of practical skills in chemistry · Practical Activity Groups · 1.2 Practical skills assessed in the practical endorsement · PAG 2: Acid-base titration
Question and mark scheme from the OCR A-Level Chemistry examination, AS Breadth in chemistry (01), November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.