Question

In a metal oxide, there is 20% oxygen by weight. Its equivalent weight is:
[A] 40
[B] 64
[C] 72
[D] 32

Answer 1

Hint: To solve this question, find the number of equivalents of oxygen and that of the metal oxide and equate them. Number of equivalents is the mass divided by equivalent weight. You can calculate the equivalent weight of oxygen by dividing its atomic weight by its valency factor.

Complete answer:
Let us consider that the total mass of the metal oxide is 100gm.
 In the question it is given to us that there is 20% oxygen by weight in the metal oxide. So, there is 20g oxygen present in the metal oxide.
Therefore, we can write that the remaining will be the metal. So there is 80% metal by weight i.e. there is 80g metal in the metal oxide.
We have to find the equivalent weight of the metal oxide.
We can compare the number of equivalents as- Mass of the metal oxide divided by its equivalent weight will be equal to the mass of oxygen divided by its equivalent weight of oxygen.
Therefore, we can write that-
$\begin{align}
  & \dfrac{mass\text{ of }metal}{equivalent\text{ wt}\text{. of }metal}=\dfrac{mass\text{ of oxygen}}{equivalent\text{ weight of oxygen}} \\
 & equivalent\text{ wt}\text{. of }metal=\dfrac{mass\text{ of metal}}{\text{ mass of oxygen}}\times equivalent\text{ weight of oxygen} \\
\end{align}$
Here, we know that the mass of oxygen is given to us which is 20g and that of metal is 80g.
We can calculate the equivalent weight of oxygen by dividing its atomic weight by its valency factor. Valency factor or n-factor for oxygen is 2.
Therefore, the equivalent weight of oxygen is 16 upon 2 which is 8.
Putting these values in the above equation will give us the equivalent weight of the metal oxide.
     $equivalent\text{ wt}\text{. of }metal=\dfrac{80}{\text{ 20}}\times 8=32$
As we can see form the above calculation that the equivalent weight of the metal oxide is 32.

Therefore, the correct answer is option [D] 32.

Note:
We should not be confused between equivalent weight and number of equivalents. Equivalent weight is the gram molecular weight of the substance divided by the valency factor of the substance. Valency factor is basically the charge it carries whereas the number of equivalents is the weight/ mass of the compound divided by its equivalent weight.