Assertion (A): One litre of hydrogen weighs the same as one litre of oxygen.

Reason (R): One litre of hydrogen contains the same number of molecules as one litre of oxygen.

1. Both A and R are true and R is the correct explanation of A.
2. Both A and R are true but R is not the correct explanation of A.
3. A is true but R is false.
4. A is false but R is true.

A is false but R is true.

Explanation — At standard temperature and pressure (STP), one litre of any ideal gas contains the same number of molecules (Avogadro’s law). But the masses differ because the molar masses of the gases are different. Hence the assertion (A) is false.

Avogadro's law states that "equal volumes of all gases under similar conditions of temperature and pressure contain the same number of molecules". This means that one litre of hydrogen contains the same number of molecules as are present in one litre of oxygen or any other gas. Hence the reason (R) is true