JEE Chemistry 2025 Question with Solution

Given: Equal mass $10^{-10} \, \text{g}$ of each element: $\text{Pb}$, $\text{Po}$, $\text{Pr}$ and $\text{Pt}$.

Find: Which element has the highest number of atoms.

The number of atoms in a given mass is directly proportional to the number of moles. Use

where $n$ is the number of moles, $m$ is mass and $M$ is molar mass.

Since the mass is the same for all the elements, the element with the smallest molar mass will have the largest number of moles, and hence the highest number of atoms.

From the solution:

• $M_{\text{Pb}} = 207 \, \text{g mol}^{-1}$
• $M_{\text{Po}} = 209 \, \text{g mol}^{-1}$
• $M_{\text{Pr}} = 140 \, \text{g mol}^{-1}$
• $M_{\text{Pt}} = 195 \, \text{g mol}^{-1}$

Thus,

The largest value is for $\text{Pr}$ because $140$ is the smallest molar mass among the given elements.

Therefore, the element having the highest number of atoms is $\text{Pr}$. The correct option is B.

Given: Same mass $10^{-10} \, \text{g}$ for all four elements.

Find: Which sample contains the maximum number of atoms.

For equal masses,

So the comparison reduces to checking only the molar masses. The smaller the molar mass, the greater the number of atoms.

Among $\text{Pb}$, $\text{Po}$, $\text{Pr}$ and $\text{Pt}$, $\text{Pr}$ has the least molar mass. Hence it has the highest number of atoms.

Therefore, the correct option is B.