Material Balances Involving Combustion

Question 1
Marks : +2 | -2
Pass Ratio : 100%
Ethene is burnt with 50% of excess air, what is the percentage of CO2 in the products?
11.11%
36.36%
66.66%
72.72%
Explanation:
The reaction is C2H4 + 3O2 -> 2CO2 + 2H2O. Basis: 10 moles of C2H4, => moles of O2 reacted = 30, => moles of O2 entered the process = 45, => moles of CO2, H2O and O2 in products are 20, 20 and 15 respectively, => percentage of CO2 = 20/55*100 = 36.36%.
Question 2
Marks : +2 | -2
Pass Ratio : 100%
What is the percentage of excess air, if 10 moles of air entered the process and only 5 moles of that are required?
10%
50%
75%
100%
Explanation:
Percentage of excess air = 100*(10 – 5)/5 = 100%.
Question 3
Marks : +2 | -2
Pass Ratio : 100%
144 grams of C5H12 is burnt with 64 grams O2, and 44 grams of CO2 is formed, what is the percentage of excess O2?
25%
50%
75%
100%
Explanation:
The balanced chemical reaction is C5H12 + 8O2 -> 5CO2 + 6H2O, => Extent of reaction = (44/44 – 0)/5 = 0.2. Moles of O2 entered the process = 64/32 = 2, Moles of O2 required = 0.2*8 = 1.6, => Percentage of excess oxygen = 100*(2 – 1.6)/1.6 = 25%.
Question 4
Marks : +2 | -2
Pass Ratio : 100%
Octane is burnt with 40% excess O2, what is the percentage of CO2 in products?
18.18%
36.36%
54.54%
72.72%
Explanation:
The reaction is C8H18 + 12.5O2 -> 8CO2 + 9H2O. Basis: 10 moles of C8H18, => moles of O2 reacted = 125, => moles of O2 entered the process = 175, => moles of CO2, H2O and O2 in products are 80, 90 and 50, => percentage of CO2 = 80/220*100 = 36.36%.
Question 5
Marks : +2 | -2
Pass Ratio : 100%
10 moles of ethane is supplied with 49 moles of oxygen, what is the percentage of excess oxygen?
20%
40%
50%
70%
Explanation:
The balanced reaction is C2H6 + 3.5O2 -> 2CO2 + 3H2O, => O2 required = 3.5*10 = 35 moles, => percentage of excess oxygen = 100*(49 – 35)/35 = 40%.
Question 6
Marks : +2 | -2
Pass Ratio : 100%
What is the percentage of excess air if 50 grams of air is in excess and 150 grams of air enters the process?
10%
25%
50%
100%
Explanation:
Percentage of excess air = 100*50/(150 – 50) = 50%.
Question 7
Marks : +2 | -2
Pass Ratio : 100%
Pentane is burnt with 100% of excess air, what is the percentage of H2O in the products?
12.4%
34.6%
42.1%
56.9%
Explanation:
The reaction is C5H12 + 8O2 -> 5CO2 + 6H2O. Basis: 10 moles of C5H12, => moles of O2 reacted = 80, => moles of O2 entered the process = 160, => moles of CO2, H2O and O2 in products are 50, 60 and 80, => percentage of H2O = 80/190*100 = 42.1%.
Question 8
Marks : +2 | -2
Pass Ratio : 100%
Propane is burnt with 20% excess O2, what is the percentage of CO2 in products?
12.5%
25%
45%
75.5%
Explanation:
The reaction is C3H8 + 5O2 -> 3CO2 + 4H2O. Basis: 10 moles of C3H8, => moles of O2 reacted = 50, => moles of O2 entered the process = 60, => moles of CO2, H2O and O2 in products are 30, 40 and 10, => percentage of CO2 = 10/80*100 = 12.5%.
Question 9
Marks : +2 | -2
Pass Ratio : 100%
The extent of a reaction is 10, and the stoichiometric coefficient of O2 is 2.5, if the moles of O2 entering the process is 45, what is the percentage of excess air?
20%
40%
60%
80%
Explanation:
Moles of O2 required = 10*2.5 = 25, => Percentage of excess air = 100*(45 – 25)/25 = 80%.
Question 10
Marks : +2 | -2
Pass Ratio : 100%
Ethene is burnt with 150% of excess air, what is the percentage of O2 in the products?
12.5%
33.3%
45.4%
52.9%
Explanation:
The reaction is C2H4 + 3O2 -> 2CO2 + 2H2O. Basis: 10 moles of C2H4, => moles of O2 reacted = 30, => moles of O2 entered the process = 75, => moles of CO2, H2O and O2 in products are 20, 20 and 45 respectively, => percentage of O2 = 45/85*100 = 52.9%.