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Question

If the atomic masses are: potassium 39 u, nitrogen 14 u, and oxygen 16 u, what is the formula unit mass of potassium nitrate (KNO3)?

This question was previously asked in
RRB Technician 2026 Grade 3 Question Paper (7-Oct-2026) (Shift 3)
The correct answer is

101 u

The formula unit mass of an ionic compound is the sum of the atomic masses of all the atoms in its formula unit, each multiplied by the number of times that atom appears.

In KNO3 there is 1 potassium atom, 1 nitrogen atom and 3 oxygen atoms. So the formula unit mass is \(1 \times 39 + 1 \times 14 + 3 \times 16\) u.

Working it out: \(39 + 14 + 48 = 101\), so the formula unit mass of potassium nitrate is 101 u.

Check: the nitrate ion \(NO_3^-\) has mass \(14 + 3 \times 16 = 62\) u, and adding potassium gives \(62 + 39 = 101\) u, the same value.

Hence, 101 u.

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