College Chemistry Students, and Professors...?

If taking an exam and I am asked what is the atomic mass of oxygen do I put 15.9994 or round it to 16?

Answer:
Depends on how many significant figures you need. If the question were "How many atoms of oxygen are in 1.00 g of oxygen?" Lets examine this..15.9994 is six sig figs, but the number 1.00 g has only three, so you can only report three sig figs in the answer, so using three in the beginning numbers shouldn't make a difference. As a rule of thumb, I usually carry two places past the decimal, so oxygen for me is 16.00 g / mole
depends on what the prof likes more...for quick calculations, such as thos on the mcat, you would you 16.
This just depends on how exact your professor wants you to be. Technically, the correct answer is 15.9994, because the atomic mass does not refer to the mass of a particular atom, but to the weighted average of the masses of all naturally occuring isotopes of that element. It just so happens that O-16 is far more abundant than any other isotope, so the atomic mass of oxygen happens to be very close to 16. But it is often rounded to 16 for simple calculations.

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