Km as Substrate Concentration at Half Vmax

Steps:

• Identify Vmax as the maximum rate on the graph's plateau.
• Calculate half Vmax (Vmax/2).
• Locate the substrate concentration [S] where the rate equals Vmax/2 on the curve.
• This [S] value is Km, reading 2 mol dm^{-3} from the graph.

Why A is correct:

• Km is defined in the Michaelis-Menten equation as the [S] at which reaction rate reaches half Vmax.

Why the others are wrong:

• B: 5 mol dm^{-3} corresponds to a rate above Vmax/2 on the curve.
• C: 30 mol dm^{-3} is a high [S] near Vmax, not half.
• D: 50 mol dm^{-3} exceeds the half-rate point, approaching saturation.

Final answer: A