Q32 - Chemistry Qbank 1

asatish
Posts: 8
Joined: Mon Aug 14, 2017 11:23 pm

Q32 - Chemistry Qbank 1

Postby asatish » Sat Sep 16, 2017 3:59 pm

Hey All,

Hoping someone could help refresh my understanding of the concept of calculating the mols/masses of atoms within a compound to get the empirical formula for the following question:

Determine the empirical formula of an unknown sugar weighing 120 g that is combusted at high pressure, yielding 176 g CO2 and 72 g H2O.

The solution finds the masses of C and H via the masses of CO2 and H2O, and then finds the mass of oxygen by subtracting by the amount of product. Can you remind me why you don't use the same method of ratios to find the moles of O within the products and relate it back to the reactant? I'm having trouble reconciling the numbers because 120 g of this sugar would have 64 g of O which is 4 moles of O. But If you did use the atomic ratios in CO2 and H2O, you technically get 8 mol of O and 4 mol of O respectively in each of your products and you clearly don't have that much in your reactants.

Sorry if this question is confusing- basically, I understand the solution by itself, but I'm not able to figure out why those values are different if you try to back-calculate for the moles of Oxygen (which I think is a gap in my conceptual understanding).

Thank you!
Anita
NS_Tutor_Andrew
Site Admin
Posts: 289
Joined: Mon May 23, 2016 1:47 pm

Re: Q32 - Chemistry Qbank 1

Postby NS_Tutor_Andrew » Mon Sep 18, 2017 11:18 am

Hi asatish,

It sounds like the core question is "why you don't use the same method of ratios to find the moles of O within the products and relate it back to the reactant?" The answer to this is that some of the O in the products comes from atmospheric oxygen, not the carbohydrate that was burned (the Wikipedia review of combustion is a good review). In contrast, there's no external source of C or H, which is why we focus on those elements in the first steps of the stoichiometric analysis. Hope this helps clarify things and feel free to reach out with any other questions that come up!
Andrew D.
Content Manager, Next Step Test Prep.
asatish
Posts: 8
Joined: Mon Aug 14, 2017 11:23 pm

Re: Q32 - Chemistry Qbank 1

Postby asatish » Mon Sep 18, 2017 4:24 pm

OH! Of course - While I processed that it was a combustion reaction, I conveniently forgot to include the atmospheric O2 into my equation when I wrote it out and started solving for the various quantities. That makes perfect sense. Thanks so much for the response!
NS_Tutor_Andrew
Site Admin
Posts: 289
Joined: Mon May 23, 2016 1:47 pm

Re: Q32 - Chemistry Qbank 1

Postby NS_Tutor_Andrew » Mon Sep 18, 2017 6:06 pm

Of course! It's all too easy to make mistakes like that but I bet now you will never forget that oxygen is part of the reactants side in combustion reactions :).
Andrew D.
Content Manager, Next Step Test Prep.

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