Propane reacts with elemental oxygen gas according to the unbalanced reaction, C3H8(g) + O2(g) → CO2(g) + H2O(l). What is the coefficient of the water, H2O(l), in the balanced reaction? a
2
b
4
c
6
d
8

b. 4

Explanation:

The reaction expression is given as:

   C₃H₈   +   O₂     →    CO₂   +  H₂O

To solve this problem, we employ a mathematical approach.

Now use coefficient a, b, c and d as the numbers that will balance the expression:

            aC₃H₈   +   bO₂     →    cCO₂   +  dH₂O

Conserving C: 3a  = c  

                   H:  8a  = 2d

                    O: 2b  = 2c + d

let a  = 1, c  = 3, d  = 4 , b  = 5

 The balanced reaction expression is:

            C₃H₈   +   5O₂     →    3CO₂   +  4H₂O

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explanation:

substituting a hydrogen atom with a halogen in a hydrocarbon:

- the single bond remains single. a is wrong.

- the bond with hydrogen is as strong as with a halogen. c is wrong

- a hydrocarbon is saturated if there is no double bond. we do not know that so d is wrong.

- carbon will always have four valence electrons. e is wrong.

by elimination, the correct answer is b. the boiling point of the new compound increases.