I’m taking the requisite Dumbass Chemistry course intended for nursing students. I’ve been staring at the first two questions from last week’s test for half an hour and I simply can’t fathom why I’m too stupid to figure it out (warning: .pdf). What am I missing here? Six moles of 0[sub]2[/sub] is 12 moles of oxygen, so the answer should be 24, right? Same logic applies to the second question, just stated differently.
By my count it would be 12 mol of O2 required, unless specifically asked about elemental oxygen.
Then again, uni was a long time ago…
I think it’s just that “oxygen” refers to O2, since you don’t ever have bulk monoatomic O.
Yes, that’s where the confusion is coming from. The equation is balanced, with 1 part Glucose and 6 parts O2. If you double the Glucose, you double the O2 to 12 parts.
Same for b), except this time you are doubling the O2.
That was my understanding too - the question refers to oxygen gas (O2). Since the original equation has 6 moles of Oxygen gas to completely react with one mole of glucose, 12 moles of oxygen gas would be required for 2 moles of glucose.
This is where you are going wrong. Six moles of oxygen means six moles of 0[sub]2[/sub]. So if you have 12 moles of oxygen, you have 12 0[sub]2[/sub]. This is double what is given in the equation, so you have to double everything else. Therefore you need two moles of glucose.
Overall the reaction would be:
2 C[sub]6[/sub]H[sub]12[/sub]O[sub]6[/sub] + 12O[sub]2[/sub]
going to
12 CO[sub]2[/sub] + 12 H[sub]2[/sub]O
For question b, the logic is the same. You are given 12 moles of oxygen, which is double what is given in the equation, so the number of moles of glucose needs to be doubled.
ETA: Rats too slow!
Your answer to part © should have clued you in to your error. You correctly answered that 12 moles of CO[sub]2[/sub] would be produced by the reaction of 2 moles of glucose; but in the equation given in the question you can see that the amounts of O[sub]2[/sub] (on the left) and CO[sub]2[/sub] (on the right) are equal. In other words, 1 mole of oxygen consumed in the reaction produces 1 mole of carbon dioxide.
So you correctly state that there will be 12 moles of CO[sub]2[/sub] on the right-hand side of the equation, but somehow doubled that to 24 moles of O[sub]2[/sub] on the left.
Your error, as others have pointed out, is thinking that 1 mole of O[sub]2[/sub] is 2 moles of “oxygen”. It isn’t. Moles refer to quantites of individual entities, and the basic unit of oxygen gas is the O[sub]2[/sub] molecule.
However, being pedantic, the question should, technically speaking, specify “How many moles of oxygen molecules” to avoid the possibility of confusion. My dictionary defines a mole as:
(emphasis added)
The phrasing would be “moles of molecular oxygen” but, since the form used in the equations is molecular oxygen throughout, there is no need to specify.
If we were really being pedantic, we’d ensure we weren’t trying to make molecules out of numbers.
(0 and O are distinct.)
(Not that you confused them, Colophon, but others in this thread did.)
Speaking of numbers, that’s all a mol is: It’s approximately 6.0210[sup]23[/sup]. One mol of something is about 6.0210[sup]23[/sup] of that thing; it could be oxygen molecules or Petticoat Junction DVDs. It’s defined in terms of mass because, well, you try to count out individual atoms on a large scale! Every atom of a given isotope weighs the same, so if you have a given mass (in grams) of a given isotope, you divide the mass you have by the mass of the isotope (in amus) to get how many mols you have. If, as is more common, you have a sample composed of multiple isotopes, it’s more complex; essentially, you have to guess what percentage of the total mass is due to each isotope, based on relative abundances, and work from there.