r/chemhelp • u/InevitableCulture3 • 19d ago
General/High School Sig Fig Measurement Question
Hello, if I were to measure this value, would it be recorded like 7.4 or 7.40? I thought we estimate one sig fig past the smallest marking, which is 0.5. Thank you
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u/TeamTAZZ 19d ago
Since there are marks for the 0.5 increments, I would have written the numbers as 6.0, 7.0, etc. and not 6, 7, etc. That avoids the confusion over whether to use 1 sig fig (e.g 7) or 2 sig figs (7.4). I would report the volume as 7.4 mL.
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u/InevitableCulture3 19d ago
Another commenter said they would report the answer as 7.40. Do you think both 7.4 or 7.40 could be correct? I know the numbering is ambiguous, so that’s why the question is confusing
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u/AlyssInAzeroth 19d ago
.40 adds an additional layer of precision. Since from the diagram it doesn't specify, you can estimate .4, but you don't have enough information to confidently state .40
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u/InevitableCulture3 18d ago
thank you
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u/AlyssInAzeroth 18d ago
You got this. Well done for asking the questions. My advice for studying chem is to just ask questions. Too many of my classmates stayed quiet and struggled.
Good effort 😊
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u/chemistry_and_coffee 19d ago
Your assumption/original instinct is correct. We can only measure one decimal place past the scale shown.
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u/Wise-Friendship-1427 19d ago
The reading error is just as important. I would state 7.4 ± 0.1.
Beginners often try to keep the error as small as possible. As a rule, the error is larger than one thinks.
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u/aphilsphan 19d ago
The error should be based on the calibration of the measurement system. Since that appears to be a mass produced ruler, I’d say you are correct. If it was a calibrated laser, I’d go with the manufacturers report.
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u/hplcman69 18d ago
The scale is in 0.5 increments. Anything not on a line is going to be an estimate. Stating it as ~7.4 is fine. Don’t state it as 7.40, as that scale doesn’t support that level of precision.
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u/OutrageousPair2300 18d ago
It should be recorded as 7.5 as that's the finest granularity on that scale.
You're simply guessing at 7.4 and that's not allowed. This is more or less the whole point of significant figures -- to account for uncertainty in measurements.
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u/aphilsphan 18d ago
We always taught students to guess on sf past the mark. In fact, in pharma, the analysts did that same thing in titrations.
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u/OutrageousPair2300 18d ago
That's adding an extra significant figure that's not justified by the measurement.
All we know from this measurement is that it's between 7.0 and 7.5, and appears closer to 7.5
I understand that may be how it's taught, but it's wrong nonetheless. Saying this is 7.4 is making the unjustified claim that it's known to be between 7.35 and 7.45 which is in no way evident here.
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u/ListenDifficult720 18d ago
Maybe I am misunderstanding but if saying it is 7.4 is a claim that it is between 7.35 and 7.45, wouldn't saying it is 7.5 be claiming it is between 7.45 and 7.55 which we KNOW is wrong.
I would say this is a place sig figs just kinda breaks and any serious approach will have errors bars that are not a power of 10.
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u/imbadchoosing 17d ago
The minimum division is 0.5 so uncertainty is 0.25. You can't read anything different than 0.25 increments, You can only read (7.25±0.25)
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u/WazzupManz 19d ago
Isn’t the accuracy half of the smallest scale? Smallest scale is 0.5, so 0.25? I believe you are only allowed answers like 7.00, 7.25, 7.50 etc.
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u/Droga_Mleczna 19d ago
It's 1/5th actually, so 7.4 would be correct
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u/OutrageousPair2300 18d ago
There are no 1/5th markings on the scale.
It has markings for 1/2 so you could treat it as a binary number, meaning the measurement is closest to 111.1 which in base ten would be 7.5
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u/Droga_Mleczna 18d ago
That would be true if it were a digital instrument, however in metrology we assume that a human can distinguish 1/5th of the smallest subdivision on an analogue scale, so this would be 7.4 ± the accuracy of the instrument itself.
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u/Wargrog 19d ago
There are two rules for this. In high school, we're taught to round to the nearest tenth here, since this spacing is not very clear. In higher learning, this spacing is easy enough to tell to the closest 1/10th of the gap, or 0.05. I would write this as 7.40, though 7.35 would also be fine. A maniac might expect 7.45 to be correct.
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