Example 1 · level 1
Calculate the formula unit mass of sodium hydroxide (NaOH) in u. Atomic masses: Na = 23 u, O = 16 u, H = 1 u.
Show solution
- Na: 1 × 23 = 23
- O: 1 × 16 = 16
- H: 1 × 1 = 1
- Total = 40 u.
Answer: 40 u
CLASS 9 SCIENCE · ATOMS AND MOLECULES
Molecular mass and formula unit mass
Add the atomic masses of all atoms in the formula (count subscripts carefully). For ionic compounds this sum is called the formula unit mass.
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Worked examples
Example 2 · level 2
Calculate the molecular mass of nitric acid (HNO₃) in u. Atomic masses: H = 1 u, N = 14 u, O = 16 u.
Show solution
- H: 1 × 1 = 1
- N: 1 × 14 = 14
- O: 3 × 16 = 48
- Total = 63 u.
Answer: 63 u
Example 3 · level 3
Calculate the molecular mass of nitric acid (HNO₃) in u. Atomic masses: H = 1 u, N = 14 u, O = 16 u.
Show solution
- H: 1 × 1 = 1
- N: 1 × 14 = 14
- O: 3 × 16 = 48
- Total = 63 u.
Answer: 63 u