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I save rounding to the end but if I see that the numbers are close (say oxygen is .47 mol and hydrogen is .51 mol and carbon is .23 mol) I just kind of guess that they mean for it to be the whole numbers (2:2:1). This might not be helpful...could you give an example of a problem like this?
Yeah, as long as you have about 3 sig figs, then the mole ratios should come out right when you use them later on to find something like the empirical formula of something. However, remember to use the right number of sig figs if the problem specifically asks for the molar ratios, but as long as the ratios are a just a part of your work on the path to finding the answer to the problem then you should be fine.
I usually don't round until the end if possible, but if I do round, I try to keep at least 3 or more sig figs. It's best to not round if possible, but rounding a little bit should not throw your answer off entirely, since the moles of each element are usually just one step towards the final answer, and even with them rounded, you can still find the ratio.
Normally for answers for problems in this class, we won't be getting really weird and large mole numbers, it will normally be something like 2:2:1, so if you get some really weird combo like 13:27:6 you may have done something wrong in rounding or in the calculations, but usually rounding will be able to get you close enough.
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