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Mistake in the 2009 Midterm Solutions?

Posted: Mon Oct 31, 2016 2:33 pm
by Tycho_Meimban_2B
For the 2009 Midterm for the "yellow book", is there an error in how the solution is solved? For example, the solution states that to find the molar ratio of the C, H, and O atoms, the book says to "divide each amount of moles by 0.2939". However, when dividing the moles of H20 and CO2, which the yellow book solves for us, the molar ratio does not come out to 1.4:1.2:1. But when the moles of each atom (moles of H, moles of C, which are not calculated in the yellow book) in the formula are divided by "0.2939", the ratio does come out.

I know this might be an unintentional error, so sorry if I'm sounding a bit annoying here. :(

Re: Mistake in the 2009 Midterm Solutions?

Posted: Mon Oct 31, 2016 3:55 pm
by Emily Wang 1H
I think what you might've done is divide the grams of C, H, and O by 0.02939 moles. For example, if you convert 0.4942 g Carbon to moles, you get 0.04114904 moles C, and if you then divide that by 0.02939, you do get 1.4. The ratio comes out to be 1.4:1.2:1 once you convert grams to moles first before dividing.

Re: Mistake in the 2009 Midterm Solutions?

Posted: Mon Oct 31, 2016 8:21 pm
by Julia Nakamura 2D
Adding onto the the comment above, whenever you are finding empirical/molecular formulas, you must first convert the elements to moles before dividing by the one that has the smallest number of moles. You can make sure to do this by seeing if the units cancel when you multiply the grams by grams/mole.