Check pipetting accuracy and precision by mass
Six weighings tell you two separate things about your technique. Learn which one says close to right and which one says the same every time.
- Weighing water stands in for measuring it: A milliliter of water has a mass of about a gram, so a balance can check a volume you have no other way of measuring. The word about is doing real work there, since the exact figure shifts a little with temperature.
- One trial shows nothing: A single weighing cannot tell you whether a result is typical. Six can, which is why the check is built as a set of repeats rather than as one careful attempt.
Prerequisites are inferred: pending teacher review.
Re-learn the skill with worked practice and clear examples.
Now read the pattern in a set of results and say what kind of problem it points at.
Six trials read 0.243, 0.241, 0.244, 0.242, 0.243 and 0.241 grams against a target of 0.200. What is the most likely cause?
Reviewed- A.Water that was colder on some of the six trials than on others
- B.A hand that shakes by a different amount on each trial
- C.A balance that has not been given time to settle between trials
- D.A fixed setting that is wrong by the same amount every time
Show the worked solution ▾
Answer: D. A fixed setting that is wrong by the same amount every time
- Step 1: Measure the spread: The six values sit within a few thousandths of each other, so the technique repeats very well.
- Step 2: Measure the offset: Every value is high by about the same amount, so the whole set is shifted in one direction.
- Step 3: Name what behaves that way: Only something fixed in place can produce the identical shift six times running.
Why it's right: The values repeat within a few thousandths of a gram, so the hands are steady. All six sit high by nearly the same amount, and only something fixed can shift every trial by the same amount. Look at the dial and the stroke, not at the student.
- A: Temperature shifts the mass of a milliliter of water very slightly. It cannot account for a difference this large, and it would vary rather than repeat.
- B: An unsteady hand produces a different error each time, which would show up as scatter. This set has almost none.
- C: A balance that has not settled gives drifting or jumping readings, not six readings that agree with each other and disagree with the target.
Aligned to Handling, Preparation, Storage and Disposal · reading level ~grade 9
A student writes, "I was precise but not accurate." Which set of results would justify that sentence?
Reviewed- A.Results that agree with each other and sit away from the target
- B.Results that scatter widely around the correct target
- C.Results that are all different and all close to the target
- D.Results that match the target on three of the six trials
Show the worked solution ▾
Answer: A. Results that agree with each other and sit away from the target
- Step 1: Translate precise: Precise is about the trials agreeing with one another.
- Step 2: Translate not accurate: Not accurate is about the trials sitting away from the target.
- Step 3: Put the two together: A set that satisfies both descriptions is a tight group in the wrong place.
Why it's right: The sentence makes two separate claims. Trials agreeing with one another is the first, and sitting away from the target is the second. Only a tight group in the wrong place satisfies both at once.
- B: Wide scatter is the opposite of the first half of the sentence, whatever the target is doing.
- C: This describes trials that are near the target but not near each other, which reverses both halves of the sentence.
- D: Hitting the target on some trials and missing on others describes an inconsistent set, so neither half of the sentence fits.
Aligned to Handling, Preparation, Storage and Disposal · reading level ~grade 9
- A student reads their own six trials, decides whether the problem is fixed or varying, and knows which one to hunt for first.
Fill these in as you work through the lesson.
- Accuracy (how close to the target):
- Precision (how close to each other):
- Systematic error (the same mistake every trial):
- Random error (a different wobble every trial):
Accuracy compares your results to the . Precision compares your results to . Six trials that cluster tightly in the wrong place are but not .
- Explain how a set of results can be precise and still be useless.
- Describe what the spread of six weights tells you that their average hides.
- Say why weighing water is a fair way to check a volume.
Six trials come out at 0.212, 0.211, 0.213, 0.212, 0.214 and 0.212 grams against a target of 0.200. The spread is tiny, so the technique repeats well. Every value is high by about the same amount, so something is set wrong rather than done unevenly. Precise, not accurate, and the fix is in the setup.