Question with Reaction rate with NO2 and H2O?

Question

Consider the oxidation of ammonia NH3 by oxygen O2 to form nitrogen dioxide NO2 and water H2O :
4NH3(g) + 7O2(g) ® 4NO2(g) + 6H2O(l)

Suppose that in this reaction water is being formed at a rate of 13.6 moles liter-1 sec-1. At what rate (in moles liter-1 sec-1) is oxygen used up?

Answer 1

For every 6 moles of water formed, 7 moles of O2 are used up.

Reaction rate can be expressed as :
-1/7(d[O2]/dt) = 1/6(d[H2O]/dt)

So ,
Rate at which O2 is used up=7/6(rate at which water is formed)
=(7/6)*13.6 mol/L/s
=15.87mol/L/s approx.

Answer 2

The units for the water formation rate is incorrect. As water in the liquid state, its concentration is constant, so the rate of water formation should be moles per second, not moles per liter per second. Likewise oxygen is assumed to be at a fixed concentration in most chemical reactions, and its reaction rate should be in moles per sec as well.

Assuming the rate of formation of water is 13.6 moles per sec, oxygen is being used up at 7/6 * 13.6 or 15.9 moles per sec

Answer 3

15.87 moles liter-1 sec-1.