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 - log _{10} (%T)* ........(1)

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

A = absorption

e = molar absorptivity

c = concentration

L = path lengt*h*

*%T = transmission*

*Using*

*A = 2 - log _{10} (%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 10^{3} mg/L

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

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