How do you calculate rate of reaction?

In a kinetics experiment, calculate the initial rate "v" of this reaction; as follows?

• In a kinetics experiment, 0.05 mL of enzyme is assayed for 2 min and a change in absorbance of 0.22 units is observed. Assuming that the reaction volume was 3 mL and that the molar absorptivity of the product is 6,390 litres/mol.cm, calculate the initial rate "v" of the reaction.

• Answer:

  v = ΔC / Δt but A = ε*b*C, so ΔA = ε*b*ΔC and ΔC = ΔA / ε (assuming b = 1 cm) so v = ΔA / (ε*Δt) = 0.22 / (6390*120) = 2.87*10^-7 mol L^-1 s^-1 = 1.72*10^-5 mol L^-1 min^-1