Which one of the following correctly represents the concentration ter…

The correct answer is Option (3) → \(\text{ppm}=\frac{\text{Number of parts of the component}}{\text{Total number of parts of all the components of the solution}}\times10^{6}\)

When a solute is present in trace quantities, its fraction is a very small number, so it is multiplied by a large factor to give a convenient value. The fraction of the component (by mass, by volume or in moles) is the same in each case; only the factor changes:

• fraction × 100 = percentage (parts per hundred)

• fraction × 10⁶ = parts per million (ppm)

For example, sea water containing 5.8 g of dissolved oxygen in 10⁶ g has 5.8 ppm of oxygen.

Why the other options are incorrect

• Option (1) \(\text{ppm}=\frac{\text{Number of parts of the component}}{\text{Total number of parts of all the components of the solution}}\) — Incorrect. Without a multiplier this is just the fraction (mass fraction or mole fraction), a number smaller than 1.

• Option (2) \(\text{ppm}=\frac{\text{Number of parts of the component}}{\text{Total number of parts of all the components of the solution}}\times10^{12}\) — Incorrect. Multiplying by 10¹² gives parts per trillion, not per million.

• Option (4) \(\text{ppm}=\frac{\text{Number of parts of the component}}{\text{Total number of parts of all the components of the solution}}\times10^{3}\) — Incorrect. Multiplying by 10³ gives parts per thousand.

NCERT (2026-27 edition):
Parts per Million

Note: The same/similar question has been asked in different years by NTA as per the details given below: