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### Homework Question

Posted: Fri Apr 06, 2018 11:53 am
Question: Vanillin is found in vanilla extracted from Mexican orchids. A report on the analysis of vanillin listed the mass percentage composition as 63.15% C, 5.30% H, and 31.55% O. In what atom ratios are the atoms present in vanillin?

I started by converting each mass to an amount of atoms
and get 5.26 mol C, 5.26 mol H, and 1.97 mol O.

I divide by the smallest of the three(1.97)and get:
C: 2.66, H: 2.66, and O: 1

My question after this is if I can round those 2.66 to 3 or if I should multiply them by 3.
Is there a way of telling when to multiply or round?

### Re: Homework Question

Posted: Fri Apr 06, 2018 11:59 am
I believe that you have to multiply by 3 because it is not close enough to 3. I am pretty sure it has to be greater than 75 to round up.

### Re: Homework Question

Posted: Fri Apr 06, 2018 12:05 pm
You would multiply by three in order to get to the closest full number and you would have to multiply the one by three too, giving you 8,8, and 3.

### Re: Homework Question

Posted: Fri Apr 06, 2018 12:10 pm
For the most part, the general rule of thumb is that they must be 75% or greater to round up. If you were to round at this point you would get the empirical formula C3H3O1, which isn’t correct. Instead you should multiple by 3 that will give you numbers that are greater than the 75% and allow you to round up and get the correct empirical formula C8H8O3.

### Re: Homework Question

Posted: Sat Apr 07, 2018 6:21 pm
You would multiply by three, because you would get a more precise answer by doing so. If you multiply 2.66 by three you get 7.98, which is a lot closer to 8 than 2.66 is to 3. Overall, these normally require a judgement call, but it is safe to say (in at least every problem that I've encountered) that when theres a number such as 1.33 or 1.66 that you're best off to multiply by three.
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