Chemistry/Significant Figures

[emmjayy, BSN, RN]

Hi, all -

I'm re-taking chemistry (doing a self-study) to prep for nursing school this fall (eeeek! Still so pumped to be able to say I'm actually GOING!!!) Anyway, I'm really having an issue with significant figures.

I know that a significant figure is a digit within a number that is either a.) a non-zero, b.) a zero in between two significant figures, or c.) a zero at the end of a number and to the right of the decimal point. All well and good. It's the rules about adding/subtracting/multiplying/dividing/unit conversions that are tripping me up. According to my textbook, you're supposed to have the same number of significant figures at the end of your addition/subtraction problem as the least precise measurement in your equation. Multiplication/division problems are supposed to result in a number that has the same number of significant figures as the measurement with the least amount of significant figures in your original equation, and unit conversions should result in a number that has the same amount of significant figures as the original measurement.

Great. Okay. But sometimes I'll do a problem (and I do the process completely correctly) but I'll write out the answer with what I think is the right amount of significant figures (e.g., 3.79 L when all of the measurements in the original equation had three significant figures) but the answer booklet will have the answer as 3.8 L. With no rational for why the writer rounded to two significant figures. It's driving me absolutely nuts because I'm performing the (fairly involved) equations completely correctly but still coming up with an answer that doesn't match the textbook.

[Larry2016]

First of all...if you are retaking the class for a better grade (and not be because you failed it), do not worry about significant figures in nursing. The only time you need to be THAT exact is in pediatrics or giving a high alert medication (like digoxin).

sig figs can get really confusing, especially when an instructor tells you your answer is wrong because you didn't use 4 decimal places for the centigrade temp (I mean who measures THAT exact...maybe NASA)

But to try to answer your question....there are some unwritten rules. For example, my instructor wanted us to use "only two decimal places for molecular weight of elements" and "treat it as an exact number." Which means exact numbers do not figure into your sig fig final answer. I always circled my numbers considered exact numbers so it wouldn't confuse me. (atmospheric pressure is an exact number, for example)

[emmjayy, BSN, RN]

First of all...if you are retaking the class for a better grade (and not be because you failed it), do not worry about significant figures in nursing. The only time you need to be THAT exact is in pediatrics or giving a high alert medication (like digoxin).

sig figs can get really confusing, especially when an instructor tells you your answer is wrong because you didn't use 4 decimal places for the centigrade temp (I mean who measures THAT exact...maybe NASA)

But to try to answer your question....there are some unwritten rules. For example, my instructor wanted us to use "only two decimal places for molecular weight of elements" and "treat it as an exact number." Which means exact numbers do not figure into your sig fig final answer. I always circled my numbers considered exact numbers so it wouldn't confuse me. (atmospheric pressure is an exact number, for example)

I'm just re-taking the course on my own because I last took chemistry in high school, which was 8 years ago for me!! So, my grade doesn't matter (although I'm doing really well aside from this one issue!) I just like knowing the how's and why's of everything because I'm rebuilding my base knowledge and while I know SF probably won't factor much into nursing, other concepts I learn in chem will (at least in terms of how fluid/electrolyte balances work) and so I really want to make sure I have all my basics down, if that makes sense.

I haven't run into exact numbers yet - I'm literally at the very beginning of the course, which for the past two modules has dealt with learning the metric system (so easy... I don't know why the U.S. doesn't use this system), doing lots of conversions to establish good habits in solving those types of problems, and learning how to do stuff like figure out density, heat capacity, etc. - lots of algebra and using significant figures. So far it seems to be designed to cement good practices and make them a second nature so that when more challenging stuff is introduced the student won't be struggling with sloppy conversions or not applying basic rules correctly.

Which leads back to my confusion as to why I sometimes see figures rounded like I described in my original post. Is there a rule that if a number can be rounded up in certain situations (like 3.79 can go to 3.8, or 1.49 x 10⁵ --> 1.5 x 10⁵)?