A Level Chemistry OCR Practice Exam

What does the term 'molecular mass' refer to?

1. The sum of the atomic masses of all atoms in a molecule

2. The mass of the most important isotope of an element

3. The mass of a single atom

4. The average mass of an element's isotopes

The correct answer is: The sum of the atomic masses of all atoms in a molecule

The term 'molecular mass' refers to the sum of the atomic masses of all atoms present in a molecule. This concept is fundamental in chemistry as it allows for the understanding of how much a given molecule weighs based on the individual atoms that compose it. Each atom has a specific atomic mass that contributes to the overall molecular mass, which is usually expressed in atomic mass units (amu) or grams per mole (g/mol). This calculation is essential for stoichiometry in chemical reactions, where knowing the molecular mass helps in determining the quantities of reactants and products involved. It also plays a critical role in calculating concentrations and in various other applications in chemistry, such as gas laws and solution chemistry. By understanding molecular mass, students can better grasp the relationships between different chemical substances and their behavior during reactions. The other choices do not accurately describe molecular mass: one refers specifically to isotopes rather than molecules, while others detail properties related to single atoms or elements rather than the collective mass of atoms in a molecule.