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 ^{-5}M/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 ^{-4}M/s**

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