stoichiometry ?

[SaintJude]

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wtf: this is such a typical mcat question that turns stoichiometry into something really f***** up.

Anaerobic organisms are able to denitrify wet soils by the following metabolic pathway.
HNO3 → HNO2 → H2N2O2 → N2O(g) → N2(g)

If all the oxygen in the nitric acid is converted to water, how many additional equivalents of acid will be consumed during the production of 5 moles of nitrogen?

How do you do this?

And please no answers with just number (like 2x3 =6 done!), b/c i can't decipher the meaning. 😛

 

[milski]

All that you need from the nitric acid is N. You get one mole out of one mole of nitric acid but you need to moles of N to make one mole nitrogen gas (it's N2). Thus you need twice as many moles nitric acid - 5x2=10.

Am I right?

 

[SaintJude]

No, answer is 50.

If you balance the equation from HNO3 --> N2, you'll find that you need 6H20. Then b/c all oxygen converts into water, you'll then need to add 10H+.(But actually why?) And that's 1 mole of nitrogen. And so for 5 moles of nitrogen you multiply 10 x 5. But I can only explain that b/c I memorized it the explanation, don't really understand how I was supposed to know to balance the equation using only the end product.

But final equation is 10 (H+) + 2HNO3 ---> N2 + 6H2O.

 

[milski]

No, answer is 50.

If you balance the equation from HNO3 --> N2, you'll find that you need 6H20. Then b/c all oxygen converts into water, you'll then need to add 10H+.(But actually why?) And that's 1 mole of nitrogen. And so for 5 moles of nitrogen you multiply 10 x 5. But I can only explain that b/c I memorized it the explanation, don't really understand how I was supposed to know to balance the equation using only the end product.

But final equation is 10 (H+) + 2HNO3 ---> N2 + 6H2O.

You need at least 2HNO3 to get N2. Then you get 6Os, so you need to get 6H2Os on the right side. And now you're short 10H from the left side. So I guess for each mole of N2 you need two moles of HNO3 to get the nitrogen and 10 more moles of acid to dissociate to get the needed H+. That actually makes sense and matches with the "how many additional equivalents of acid" part of the question.

 

[SaintJude]

Ok, I think the main problem was that I didn't know that "equivalents of acid" was just a fancy phrase for "moles of H+" .