If a 45 mM phosphate solution (solution A) had an absorbance of 1.012. What would be the absorbance if 11 mL of solution A was used to prepare a 20.0 mL of a new phosphate solution. Write answer to three significant figures.

0.550

Explanation:

The absorbance (A) of a substance depends on its concentration (c) according to Beer-Lambert law.

A = ε. l. c

where,

ε: absorptivity of the species

l: optical path length

A 45 mM phosphate solution (solution A) had an absorbance of 1.012.

A = ε. l. c

1.012 = ε. l. 45 mM

ε. l = 0.022 mM⁻¹

We can find the concentration of the second solution using the dilution rule.

C₁. V₁ = C₂. V₂

45mM. 11mL = C₂. 20.0 mL

C₂ = 25 mM

The absorbance of the second solution is:

A = (ε. l). c

A = (0.022 mM⁻¹). 25 mM = 0.55 (rounding off to 3 significant figures = 0.550)