Molecular Mass
Molecular mass is the sum of the atomic masses of all atoms present in one molecule of a substance.
Practice This ConceptMain explanation
Teacher explanation
For a molecular substance, read the formula carefully, multiply each element's atomic mass by the number of its atoms, and then add the values. It is expressed in atomic mass units, u.
Example
Molecular mass of H2O = 2 x 1 + 16 = 18 u.
Simple analogy
Subscript means multiply.
Common confusion
Students often forget to multiply the atomic mass by the subscript, especially in formulae like CO2 or H2SO4.
Exam tip
Underline each subscript and multiply before adding; do not add atomic masses only once.
Answer writing and exam use
1-mark use
Write the exact meaning of molecular mass in one clean line.
2-mark use
Define molecular mass and add one example or condition.
3-mark use
Explain molecular mass, show the method or example, and mention the common mistake.
Practice this concept with focused MCQs
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