JEE Main 2025 — Some Basic Concepts Of Chemistry Question with Solution

To determine which element has the highest number of atoms among 10^-9 grams of the elements Pb, Po, Pr, and Pt, we use the formula for calculating the number of atoms:

$\text{Number of atoms}=\frac{\text{Mass}}{\text{Molar Mass (g/mol)}}\times N_A$

where $N_A$ is Avogadro's number.

From this formula, it's clear that for a given mass, the element with the smallest molar mass will have the greatest number of atoms, as it appears in the denominator.

Below are the molar masses of the elements:

Molar mass of Pr (Praseodymium), $M_{\text{Pr}}=141$ g/mol

Molar mass of Pt (Platinum), $M_{\text{Pt}}=195$ g/mol

Molar mass of Pb (Lead), $M_{\text{Pb}}=207$ g/mol

Molar mass of Po (Polonium), $M_{\text{Po}}=209$ g/mol

Since Praseodymium (Pr) has the smallest molar mass ($M_{\text{Pr}}=141$ g/mol), 10^-9 grams of Pr will contain the highest number of atoms compared to the other elements listed.