Question

Calculate the concentration in ppm of a K2CrO4 solution that has a %T of 77.17 in a 4.50cm cell at 372nm, the molar absorptivity is 4,459.0 L mol-1 cm-1. [Report the result to two decimal places]

Answer 1

There are two equations

A = 2 - log₁₀ (%T) ........(1)

A = e c L ........(2)

A = absorption

e = molar absorptivity

c = concentration

L = path length

%T = transmission

Using

A = 2 - log₁₀ (%T)

    = 2 - log (77.17)

     = 0.11255

Putting this value in equation (2)

A = e c L

c = A/eL

   = 0.11255/(4459.0x4.50)

    = 5.609 x 10^-6 moles/Litre

Molecular mass of K2CrO4 = 194.2 g/mol

so, c = 5.609 x 10^-6 mol/L x 194.2 g/mol

         = 1.089 x 10^-3 g/L

         = 1.089 x 10^-3 x 10³ mg/L

          = 1.09 ppm          [1 ppm = 1 mg/L]