Final answer:
Two samples of molecular hydrogen can have the same number of atoms but different molecular weights because they may consist of different isotopes of hydrogen, such as protium and deuterium, which have different atomic masses.
Step-by-step explanation:
To understand how two samples of molecular hydrogen can contain the same number of atoms but have different molecular weights, we must consider isotopes. Hydrogen has three isotopes - protium (1H), deuterium (2H), and tritium (3H). Protium, the most common isotope, has a mass of approximately 1 amu (atomic mass unit). Deuterium, the heavy hydrogen, has one neutron, giving it a mass of approximately 2 amu. Similarly, tritium has two neutrons with a mass of about 3 amu. If one sample of hydrogen gas (H2) comprises molecules made of two protium atoms, and another sample is made of two deuterium atoms, they would contain the same number of hydrogen atoms but differ in molecular weight. It's like having the same number of balls with different weights - while the count is the same, the total weight varies.
Isotope Example
Let's take two diatomic hydrogen molecules: one molecule of 1H2 (which is about 2 amu) and one of 2H2 (which is about 4 amu). Even though each molecule contains two hydrogen atoms, the deuterium-based molecule weighs twice as much as the protium-based one because of the neutron in deuterium. Therefore, two samples with the same number of hydrogen molecules can have different molar masses when composed of different isotopes.