Examkracker 30 minute exam chemsitry lecture 1

[Turtlespeed]

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Question 19 states that "if 1 mole of C3H8 is reacted with 2.5 moles of O2, how many moles of H2O would be produced"


A. 1 mole of H2O
B. 2 mole of H2O
C. 3 mole of H2O
D. 4 mole of H2O

The correct answer is B, saying that O2 is the limiting reactant. Why would not 4 moles be produced since 2.5 mole of O2 could make 5 moles of H2O??

 

[loveoforganic]

CnH2n+2 + (1.5n+0.5)O2 → (n+1)H2O + nCO2

Formula for the complete combustion of an alkane^

1.5n+0.5 = 2

1.5n = 1.5

n = 1

1+1 = 2 mole H2O

 

[lynn86]

Question 19 states that "if 1 mole of C3H8 is reacted with 2.5 moles of O2, how many moles of H2O would be produced"


A. 1 mole of H2O
B. 2 mole of H2O
C. 3 mole of H2O
D. 4 mole of H2O

The correct answer is B, saying that O2 is the limiting reactant. Why would not 4 moles be produced since 2.5 mole of O2 could make 5 moles of H2O??

C3H8 + O2 -> CO2 + H2O + HEAT

If you balance this equation it becomes:

C3H8 + (5)O2 -> (3)CO2 + (4)H2O + HEAT

Notice that I mole of C3H8 reacts with 5 moles of O2.
Therefore, if only 2.5moles of O2 are available, this means that only half of the C3H8 will react. Thus O2 is your limiting reagent because 2.5 moles O2 will only react with 0.5 moles C3H8 therefore leaving the other 0.5 moles C3H8 unreacted.

If you want to find number of moles of H2O produced, first look at the balanced equation and determine the original molar ratio of the reaction. Which is:
C3H8 + (5)O2 -> (3)CO2 + (4)H2O + HEAT

C3H8 : O2 -> CO2 : H2O
1 : 5 -> 3 : 4 - combustion for balanced reaction

0.5 : 2.5 -> 1.5 : 2 - combustion for 2.5 moles O2

Hope this helps.

 

[Turtlespeed]

Thanks a lot, I forgot about the CO2.

 

[370096]

This may be a dumb question, but how do we know that the equation doesn't make CO in stead of CO2?

 

[apumic]

This may be a dumb question, but how do we know that the equation doesn't make CO in stead of CO2?

Basic gen chem knowledge. It's the combustion rxn for an alkane. CO2 & H2O are the main products. If you review your notes from gen chem or a review book, you'll see there are some rxns you should simply know.

You could also figure this out from your bio knowledge (i.e., combustion of glucose as taught in any 100-level gen bio class).