Chemistry 5070/12 — May/June 2017

Walkthrough

The question asks which apparatus must always be filled to the same level. Let us look at each option:

• A (burette): A burette is used in titrations to deliver a variable volume of liquid. It has a tap at the bottom and markings along its length (usually 0.00 cm³ at the top to 50.00 cm³ at the bottom). Before use, it is filled to near the 0.00 mark, but the volume delivered is calculated from the difference between initial and final readings. It does not have a single fixed level it must be filled to for every use, though standard practice is to fill to 0.00.
• B (gas syringe): A gas syringe is used to collect and measure the volume of a gas produced in a reaction. As gas is produced, the plunger is pushed out, so the level of the plunger changes continuously.
• C (measuring cylinder): A measuring cylinder is used to measure a rough volume of liquid. You pour liquid into it until it reaches the desired mark. The level depends on how much liquid you want to measure, so it is not always filled to the same level.
• D (pipette): The diagram shows a volumetric (or graduated) pipette. It has a bulb in the middle and a single calibration mark on the narrow upper stem. It is designed to measure one specific, fixed volume of liquid (for example, 25.0 cm³). To use it correctly, you must fill it until the bottom of the meniscus sits exactly on that single mark. Therefore, it is always filled to the same level.

Key Takeaways

• Volumetric pipettes are used to measure a single, fixed volume precisely. They have one calibration mark and must always be filled to that mark.
• Burettes, measuring cylinders, and gas syringes are used to measure variable volumes; their contents change during use.

Common Mistakes

• Confusing a burette with a pipette. While a burette is typically filled to the 0.00 cm³ mark before a titration, its defining feature is that it measures a variable volume delivered (final reading minus initial reading). A volumetric pipette is defined by having only one mark that must be filled to.
• Mistaking the volumetric pipette (D) for a Pasteur pipette (dropper). A Pasteur pipette has no markings and is used to add drops of liquid. The apparatus shown has a bulb and a single mark, identifying it as a volumetric pipette.

Things to Be Careful About

• Ensure you can distinguish the shapes of the apparatus: a burette is long and thin with a tap; a measuring cylinder is wide with a spout; a gas syringe is horizontal with a plunger; a volumetric pipette has a bulb in the middle and a single mark on the upper stem.
• In titrations, a burette is filled to the 0.00 cm³ mark, but the question asks which apparatus should always be filled to the same level to measure its designated volume. A pipette's entire purpose is to deliver a fixed volume, requiring it to be filled to its single mark every time.