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So i had my first chemistry 101 exam and got 21/25 , but iam not sure about one question, it's about significant digits , can u guys check if my answer is true or false as shown in picture. Thanksenter image description here

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closed as off-topic by Mithoron, ron, airhuff, Tyberius, andselisk Nov 16 '17 at 22:58

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  • $\begingroup$ We generally try to avoid just checking answers on this site. Usually, we want to see what your thoughts on the question were/what research you have done to try to arrive at an answer. $\endgroup$ – Tyberius Nov 16 '17 at 16:59
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Truthfully this seems like a poor question. Either (A) or (B) could be argued.

(A) The ruler is marked in cm, so the measurement can only be made guaranteed to the cm. You can try to interpolate to get something better that a whole cm, but there is no guarantee that you'll be able to interpolate to 0.1 cm. It might be +/- 0.2 cm, or +/- 0.3 cm. So you get 1+ significant figures, but not quite 2.

(B) Although the ruler is marked in cm, the measurement can be interpolated to 0.1 cm. So there are 2 significant figures.

Now since (A) was marked incorrect, the teacher was obviously looking for (B). Such interpolation is not uncommon in a chemistry lab. For instance in reading a burette.

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    $\begingroup$ I'd like to augment this answer by saying that the ruler shown is clearly not to scale. Therefore, I have no idea how accurate the 1 centimeter ticks are even. In the absence of any other context, I don't think we can assign any kind of confidence to the base measurement let along try to interpolate a measurement to 0.1 cm. The context is really important here. $\endgroup$ – Zhe Nov 16 '17 at 18:54
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It looks like the answer should be B. A good rule of thumb to follow is that your number of significant digits goes one past what is directly marked on your ruler, graduated cylinder, etc. The significant digit should be the one which has some uncertainty. Choosing A implies that the error in your measurement is on the order of $\pm1\pu{cm}$ whereas B correctly shows that you can make a reasonable measurement on the order of $\pm0.1\pu{cm}$.

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