Question

If the absorbance of a solution of copper(II) ion decreases by 28.00% upon dilution, what volume of water was added to 20.0 mL of a 1.20 M solution of Cu2+(aq)?

Answer 1

Absorbance and concentrations are linearly related. That is

A = e*l*c

Where A is absorbance

l is path length

c is concentration

e = molar absorptivity

Here:

A2 = (100-28)% of A1

= 72 % of A1

So,

A2/A1 = 0.72

So,

A2/A1 = C2/C1

0.72 = C2/1.20

C2 = 0.864 M

use dilution formula

M1*V1 = M2*V2

1---> is for stock solution

2---> is for diluted solution

Given:

M1 = 1.2 M

M2 = 0.864 M

V1 = 20 mL

use:

M1*V1 = M2*V2

V2 = (M1 * V1) / M2

V2 = (1.2*20)/0.864

V2 = 27.8 mL

volume of water added = V2 - V1

= 27.8 mL - 20.0 mL

= 7.8 mL

Answer: 7.8 mL