Question

For given hypothetical elementary parallel reaction, where $\frac{k_1}{k_2}=\frac{1}{2}$, initially only $2$ moles of $A$ are present. The total number of moles of $A,B$ and $C$ at the end of $75$% reaction are :

• A. $2$
• B. $3$
• C. $4$
• D. $3.5$

Answer 1

Answer: (D)

$3.5$

Initial number of moles of A = 2.
The reaction is 75% complete.
The number of moles of A that has reacted $=2\times \frac{75}{100}=1.5$ moles.
The number of moles of A that has remained unreacted $=2-1.5=0.5$ moles.
Now,

we have, $\frac{k_1}{k_2}=\frac{1}{2}$

Hence, the number of moles of A that has reacted to form C is twice the number of moles of A that has reacted to form B.
Hence, out of 1.5 moles of A, 0.5 moles of A has reacted to give $2\times 0.5=1$ mole of B.
Also, 1 mole of A has reacted to give $2\times 1=2$ moles of C.

Total number of moles present after the 75% completion of the reaction is 0.5 moles A $+$ 1 mole B $+$ 2 moles C $=$ 3.5 moles.