Chemistry Q51 – JEE-MAIN 2025

Home Chemistry Basic ChemistryQuestion #69

Among

10^{- 9}

g (each) of the following elements, which one will have the highest number of atoms? Element: Pb, Po, Pr and Pt

Clue 1 of 3: Fundamental Relationship

The number of atoms in a given mass of an element is related to its molar mass.

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Ninja Strategy•Inverse Relationship with Molar Mass

Recognize that for a fixed mass, the number of atoms is inversely proportional to the atomic mass, so the element with the smallest atomic mass will have the most atoms.

🧠 Full Solution Path

Step 1: Relate Number of Atoms to Molar Mass✦ Active

The number of atoms ( $N$ ) in a given mass ( $m$ ) of an element is calculated using the formula $N = \frac{m}{Molar Mass} \times N_{A}$ , where $N_{A}$ is Avogadro's number. Since the mass ( $m = 10^{- 9}$ g) and Avogadro's number ( $N_{A}$ ) are constant for all elements, the number of atoms is inversely proportional to the molar mass. Therefore, the element with the lowest molar mass will have the highest number of atoms.

Step 2: Identify Molar Masses of Elements○ Expand

Let's find the approximate atomic masses (molar masses) for each given element:

\begin{array}{ll} Pb (Lead) & \approx 207.2 g/mol \\ Po (Polonium) & \approx 209 g/mol \\ Pr (Praseodymium) & \approx 140.9 g/mol \\ Pt (Platinum) & \approx 195.1 g/mol \end{array}

💡 Teacher's Secret Hint

Recall the approximate atomic masses from the periodic table.

Step 3: Determine Element with Highest Number of Atoms○ Expand

Comparing the molar masses, Praseodymium (Pr) has the lowest molar mass ( $\approx 140.9$ g/mol) among the given options. Consequently, for the same mass of $10^{- 9}$ g, Pr will have the highest number of atoms.

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Chemistry Question 51 – JEE-MAIN 2025

Clue 1 of 3: Fundamental Relationship

🧠 Full Solution Path