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When doing the process of finding the moles of each element (having to divide the mass of the element by the atomic mass/mole of an element), how do we determine how many significant figures to use? For example, molar mass of Carbon, do I use 12.01 g/mol or 12.011 g/mol? How do you differentiate when to use the 2 instead of the three significant figures?
I think that for now, the best molar masses to use, if not given in the problem, would be the ones listed in the periodic table at the beginning of the textbook. If a problem gives you a number to use for an element's molar mass, then that would probably be the best number to use. I'm not sure if a problem's values affects the number you use for the molar mass of an element, but I hope this helps.
I think it's ok to use whichever number you get from the periodic table (but the more specific the better), but your final answer should have the same number of significant figures as that of the given (that is, the numbers you're provided in the question to do the problem).
In my chemistry discussion this morning, my TA informed our class that we will be given a periodic table to use during the test. He said that it would most likely go to two significant figures. So, Carbon would be 12.01. Depends on the periodic table given. Just like the other replies above, I would say to make use of the periodic table given by the teacher or TA to be as accurate as possible!
When calculating the moles of an element using the molar mass, I believe its best to be as accurate as possible and use the value given to you in the periodic table with that number of significant figures unless otherwise stated. When you reach the final answer through division, though, your end result (in this case the number of moles) should have the same number of significant figures as the given value with the least number of significant figures. Therefore, if the question provided you with the mass of the compound as being 49.5 grams (which is three significant figures), your resulting number of moles should also have three significant figures.
You should definitely use the most accurate molar mass given. I wouldn't worry about sigfigs until the end. Either way with rounding, your answer won't be too far off from the answer you should have, but it is more accurate to use the most digits given on your periodic table for molar mass. It's best to not round for sigfigs until the end of your calculations.
Both molar masses would be adequate and likely get you the same result because the difference is so small. In general the more accurate one would be preferable. You should keep as many sig figs as possible until the very end of the calculation when you have your answer
When answering questions I would use whatever is given either by the periodic table during the test or even the question itself! It shouldn't really matter if you use the more accurate molar mass as we have to show all of our work anyway and the answers will be very similar; but the answer any solution manual, etc. would have would use given information.
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