Question

Consider the following hypothetical aqueous reaction: A(aq)→B(aq). A flask is charged with 0.065 mol of A in a total volume of 100.0 mL. The following data are collected:

Time (min)	0	10	20	30	40
Moles of A	0.065	0.051	0.042	0.036	0.031

Part A

Calculate the average rate of disappearance of A between t= 30 min and t= 40 min, in units of M/s.

Express your answer using two significant figures.

Part B

Between t= 10 min and t= 30 min, what is the average rate of appearance of B in units of M/s? Assume that the volume of the solution is constant.

Express your answer using two significant figures.

Answer 1

For the reaction

A(aq)→B(aq)

the average rate of disappearance Of A = - ΔA / Δt

ΔA = change in concentration of A = Af - Ai

Δt = tf - ti

Minus sign indicates concentration of A is decreasing.

ANSWER TO (A)

Average rate of disappearance A between t =30 min to t=40 min

ΔA = Af - Ai

Af = No. of moles of A / Volume in liters = 0.031 / 0.1 = 0.31M

because 100mL = 100 / 1000 = 0.1L

similarly Ai = 0.036 / 0.1 = 0.36M

ΔA = 0.31 - 0.36 = -0.05M

Δt = 40 - 30 = 10min = 10 X 60 = 600s

therefore average rate of disappearance of A = - (-0.05) / 600 = 8.3 X 10^-5M/s

(B) average rate of appearance of B = average rate of disappearance of A

average rate of disappearance of A between t=10 and t=30 = - ΔA / Δt

ΔA = 0.36 - 0.51 = - 0.15

Δt = 30 - 10 = 20min = 20 X 60 = 1200s

average rate of disappearance of A between t=10 and t=30 = - (-0.15) / 1200 = 1.25 X 10^-4M/s