The unit of rate constant for a zero order reaction is

The correct answer is Option (1) → mol L⁻¹ s⁻¹

The rate of a reaction is a change in concentration per unit time, so its unit is always mol L⁻¹ s⁻¹. The unit of the rate constant follows from the rate law for order n:

    \( \displaystyle \text{Rate}=k[\mathrm{A}]^n\ \Rightarrow\ k=\frac{\text{Rate}}{[\mathrm{A}]^n} \)

    \( \displaystyle \text{Unit of }k=\frac{\mathrm{mol\,L^{-1}\,s^{-1}}}{(\mathrm{mol\,L^{-1}})^n}=(\mathrm{mol\,L^{-1}})^{1-n}\,\mathrm{s^{-1}} \)

Zero order (n = 0): Rate = k[A]⁰ = k, so k has the unit of rate itself:

    \( \displaystyle \text{Unit of }k=\mathrm{mol\,L^{-1}\,s^{-1}} \)

Why the other options are incorrect

• Option (2) mol⁻¹ L s⁻¹ — Incorrect. This is the unit of k for a second order reaction.

• Option (3) mol⁻¹ L⁻¹ s⁻¹ — Incorrect. This does not fit any order; the powers of mol and L in (mol L⁻¹)¹⁻ⁿ always have opposite signs.

• Option (4) mol⁻¹ L s — Incorrect. Every rate constant contains s⁻¹, never s.

NCERT (2026-27 edition):
Units of Rate Constant

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