Combustion analysis rounding

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Kaihan_Danesh_2J
Posts: 94
Joined: Wed Sep 30, 2020 9:40 pm

Combustion analysis rounding

Postby Kaihan_Danesh_2J » Sun Nov 01, 2020 7:09 pm

For combustion analysis questions, when finding the mole ratios, do you guys round off when finding the number of moles for each element: c, h, and o.

Natalie Do 3F
Posts: 114
Joined: Wed Sep 30, 2020 10:03 pm

Re: Combustion analysis rounding

Postby Natalie Do 3F » Sun Nov 01, 2020 8:27 pm

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?

Andreas Krumbein 1L
Posts: 69
Joined: Wed Sep 30, 2020 10:00 pm

Re: Combustion analysis rounding

Postby Andreas Krumbein 1L » Sun Nov 01, 2020 8:30 pm

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.

Amanda Bueno-Kling
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Re: Combustion analysis rounding

Postby Amanda Bueno-Kling » Sun Nov 01, 2020 11:06 pm

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.

OwenSumter_2F
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Re: Combustion analysis rounding

Postby OwenSumter_2F » Wed Nov 04, 2020 10:44 am

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.

Ava Nickman
Posts: 100
Joined: Wed Sep 30, 2020 9:54 pm

Re: Combustion analysis rounding

Postby Ava Nickman » Wed Nov 04, 2020 10:47 am

I write the rounded version on my paper but keep the unrounded answer in my calculator. Then I round at the very end so its accurate. You can round if you want I think!


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