How to use a pipette. You have access to two variable volume pipettes, a 5-50µl pipette and a 50-250µl pipette. You need to precisely dispense 255µl. Select the best way to dispense this volume.

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Examining the problem: you must deliver exactly 255 µl using two different variable-volume pipettes (5-50 µl and 50-250 µl). The goal is precise dispensing by combining volumes if needed.
Option a: It is not possible to dispense 255 µl with the pipettes you have available. This is incorrect because combining a 250 µl portion from the larger pipette with a 5 µl portion from the smaller one yields 255 µl, so 255 µl is achievable.
Option b: Use the 50-250 µl pipette and set it to 127.5 µl and dispense 127.5 µl twice. This relies on dispensing an equal half twice from the same pipette. While mathematically it would reach 255 µl, many microliter pipettes are not calibrated for accurate 127.5 µl transfers in a single action, and repeating the same partial dispense can introduce cumulative error and does not take advantage of pipette accuracy across ranges. Additionally, the question emphasizes precision and correct combination of available ranges rather than split halves that may be less reliable.
Option c: Use the 50-250 µl pipette and set it to 255 µl. The pipettes are able to be set beyond the set range, and dispensing a single volume will give the most accurate volume. This is false because most pipettes have a specified nominal range (e.g., 50-250 µl). Setting beyond the upper limit is outside the instrument’s calibration and would be inaccurate or impossible, so this option misstates how these devices operate.
Option d: Use only the 5-50 µl pipette, and dispense 50 µl five times. Then adjust the pipette to 5 µl and dispense the final 5 µl. The 5-50 µl pipette will be more accurate at pipetting than the 50-250 µl. This approach adds multiple steps and relies on the smaller pipette for the majority of the volume. While the 5-50 µl may be accurate for small volumes, repeatedly dispensing 50 µl five times and then 5 µl is inefficient and increases chances of cumulative timing or tip-handling errors. It also neglects the practicality of using the larger range for larger portions, and the instruction to achieve precise total suggests a simpler, more reliable combination.
Option e: Use the 50-250 µl pipette and set it to 250 µl to dispense the first volume, then use the 5-50 µl pipette and set it to 5 µl to dispense the second volume to give a total of 255 µl. This is a sensible and accurate approach: a single accurate 250 µl transfer from the larger pipette plus a precise 5 µl addition from the smaller pipette combines to 255 µl with minimal volumetric error, leveraging the strengths of each instrument’s precision within its respective range. It also minimizes steps and potential errors from repeated or excessive handling, making it the best option among those presented.
In summary, option a is incorrect because 255 µl is achievable; option b is flawed due to relying on a precise half-volume transfer that may be unreliable; option c misstates instrument capability and calibration; option d is overly cumbersome and introduces more error risk; option e presents a straightforward, accurate combination using each pipette within its proper range. The recommended approach is to combine 250 µl from the 50-250 µl pipette with 5 µl from the 5-50 µl pipette to reach 255 µl.

BPS3071 S1 2025 Pre-lab questions for Lab 3

How to use a pipette. You have access to two variable volume pipettes, a 5-50µl pipette and a 50-250µl pipette. You need to precisely dispense 255µl. Select the best way to dispense this volume.

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