## Homework Question

Moderators: Chem_Mod, Chem_Admin

sharonvivianv
Posts: 63
Joined: Fri Apr 06, 2018 11:05 am

### Homework Question

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?

Alexandra Wade 1L
Posts: 32
Joined: Fri Apr 06, 2018 11:05 am
Been upvoted: 1 time

### Re: Homework Question

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.

Alexis Bravo 1D
Posts: 30
Joined: Tue Nov 14, 2017 3:01 am

### Re: Homework Question

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.

Jesus A Cuevas - 1E
Posts: 29
Joined: Fri Apr 06, 2018 11:02 am

### Re: Homework Question

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.

Alicia Beebe
Posts: 37
Joined: Fri Apr 06, 2018 11:01 am

### Re: Homework Question

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.

Return to “Empirical & Molecular Formulas”

### Who is online

Users browsing this forum: No registered users and 0 guests