What is the theoretical yield of NO2 ?

Question

Nitric oxide, NO, reacts instantly with oxygen gas to give nitrogen dioxide, NO2, a dark brown gas.
In one experiment 19.13 g of NO is mixed with 12.75 g of O2.

What is the theoretical yield of NO2 ? I.E., how many grams of NO2 should be formed

Answer 1

Reaction:

2NO + O2 → 2NO2

Molar masses (a.k.a. 'molecular weights') are:

NO = 30 g per mol
O2 = 32 g per mol
NO2 = 46 g per mol

Therefore you have 19,13/30 = 0,6377 moles of NO
and 12,75/32 = 0.398 moles of O2

Since 2 moles of NO reacts with 1 mol of O2, then you can see that ALL the moles of NO will react with 0,6377/2 = 0,319 moles of O2.

So there's excess oxygen.

Finally, since 2 moles of NO will produce 2 moles of NO2, then you have in terms of mass:

2 x 30 g of NO produces 2 x 46 g of NO2
19,13 g of NO will produce X

X = 19,13 * 92 / 60 = 29,33 g of NO2

Done! ^_^

Answer 2

The usual way of asking is to predict the amount of product formed from one starting material in the presence of excess other. But in this case, you have to find the limiting reagent--the one that will run out first. The balanced equation is 2NO + O2 ===> 2NO2. Atomic weights: N=14, O=16. Molecular weights: NO=30, NO2=46, O2=32.

19.13gNO x 1mol NO/30gNO = 0.6377molNO, which is about 0.64molNO

12.75gO2 x 1molO2/32gO2 = 0.3984molO2, which is about 040molO2.

0.64molNO would require 0.32 O2. There is more than 0.32molO2. There is 0.3984molO2. So NO is the limiting reagent.

Start with 0.6377molNO. Multiply by 1molNO2/2molsNO. That comes from the balanced equation. The molsNO cancel, leaving molNO2. Multiply by 46gNO2/1molNO2. That is one answer.

I don't understand what they mean by the theoretical yield. The theoretical yield is if the reaction goes 100% to completion. But they already say that the gases react instantaneously.

I can only guess that they want you to calculate again on the basis of the amount of oxygen, and then take the difference and multiply by 100%.

Answer 3

Start by balancing an equation for the reaction:

4 NO + 3 O2 = 4 NO2 + O2