Question

Now let M react, and at time = 0, [M] = 0.102 M, A(545nm) = 0.491; at time = 30 seconds, A(545nm) = 0.404. What is the [M] at 30 seconds? What is the slope of the Absorbance vs. [M] function? What is the (average) rate of the reaction during this time period.

If a new reaction was run leaving [N] the same, but beginning (time = 0) with [M] = 0.204 M, A(545nm) = 0.981. After 30 seconds, A (545 nm) is 0.807. What is the order of the reaction with respect to [M] ?

Answer 1

Part (1).

According to Beer-Lambert's law: A = [M]l

Where '' is the molar absorptivity of the medium.

'Á' is the absorbance of the solution = 0.491

[M] is the concentration of the solution.

'l' is the path length = 1 cm (In general)

At t = 0 s: 0.491 = * 0.102 M * 1 cm

i.e. = 4.814 M^-1.cm^-1

At t = 30 s: 0.404 = 4.814 M^-1.cm^-1 * [M] * 1 cm

i.e. [M] at 30 s = 0.084 M

Therefore, the slope of the absorbance versus [M], = 4.814 M^-1.cm^-1

The average rate of the reaction = (0.102 M - 0.084 M)/(30 - 0) s = 6*10^-4 M/s

Part (2).

When [M] is doubled, the absorbance is also doubled.

Hence, the order of the reaction with respect to [M] = 1