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 Posted: Wed Jun 16, 2010 10:04 am Post subject: Chapter 1: Stoichoimetry |
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admin
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| Posted: Thu Jun 17, 2010 2:29 pm Post subject: |
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| Quote: | Vitamin C problem, mg and grams
While I was reviewing Gen. Chem chap 1, an example was used to further examine the concept on the percent mass composition on page Chm-06:
Upon burning 4.0 mg sample of pure vitamin C yields the following:
6.0 mg CO2
1.632 mg H2O
then it asks to find the amount of C in this 6.0 mg of CO2
Given that the molecular weight of CO2 is 44 g/mol, the amount of C found in CO2 would be 12/44 (the units are g/mol)
Since we're given 6 mg of CO2, wouldn't it be appropriate to convert mg --> g (gram) before further calculations to keep the units?
The text however seems to ignore this step and the answer remains in mg. I was wondering was this a typo? or did the gram got canceled between 12 g/mol for C/22 g/mol CO2 and therefore we're left with mg?
If I had converted the mg to g before I continue my calculations, I would wind up with 0.00163g instead of 1.636mg
please clarify this for me.
Thank you.
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You may want to take another peek at page 55 in the GS textbook which discusses the importance of Dimensional Analysis (keeping an eye on the units; Part II, section 2.3 #16).
Using DA, we can answer "yes" to your question: "did the gram got canceled between 12 g/mol for C / 22 g/mol CO2 and therefore we're left with mg?"
And because of this, it is not necessary to convert to g (after all, you would have to convert back to mg as soon as you finish a less comfortable calculation). |
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