Question

What coefficient is needed $ {{H}_{2}}O $ to balance the following equation?
 $ {{C}_{3}}{{H}_{8}}\text{ + }{{\text{O}}_{2}}\text{ }\to \text{ C}{{\text{O}}_{2}}\text{ + }{{\text{H}}_{2}}O $
(A) 4
(B) 1
(C) 2
(D) 3

Answer 1

Hint: A balanced chemical equation is defined as the equation in which every element on the reactant side has the same number of atoms as the element on the product side. The unbalanced elements have to multiply by a certain coefficient to equalize the number of atoms

Complete answer:
The given unbalanced equation is:
 $ {{C}_{3}}{{H}_{8\text{ }}}+\text{ }{{\text{O}}_{2\text{ }}}\to \text{ C}{{\text{O}}_{2}}\text{ + }{{\text{H}}_{2}}O $ .
The three elements that have to be balanced are Carbon (C), Oxygen (O), and Hydrogen (H).
Let us see the number of atoms of each element on the reactant side and the product side.

Element	Number of atoms on the reactant side	Number of atoms on the product side
Carbon (C)	3	1
Oxygen (O)	2	3
Hydrogen (H)	8	2

All three elements have a different number of atoms on both sides of the reaction. These elements have to be balanced by multiplying them by a suitable co-efficient.
Firstly, let's balance Carbon, multiplying the carbon on the product by 3 will equalize the no of atoms of carbon.
After placing coefficient 3 with CO2, the number of oxygen atoms changes.
Now, we’ll balance the Hydrogen atoms. Multiply the H2O on the product side with 4.
Finally, balance the Oxygen atoms on the reactant side by placing the coefficient 5 with O2.
Thus, the final balanced equation is:
 $ {{C}_{3}}{{H}_{8}}\text{ + 5}{{\text{O}}_{2}}\text{ }\to \text{ 3C}{{\text{O}}_{2}}\text{ + 4}{{\text{H}}_{2}}O $
Final answer is option A- 4.

Note:
While multiplying the elements with coefficients to equalize the number of atoms in the equation, always remember to multiply the coefficient with the associated subscript of the element. Only then will the number of atoms be the same.