PNP (2.8 mg) (MW = 140) was dissolved in 20 mL of 0.1 M NaOH. A small aliquot of

Question

PNP (2.8 mg) (MW = 140) was dissolved in 20 mL of 0.1 M NaOH. A small aliquot of this solution (0.1 mL) was diluted with 4.9 mL of pH 8 buffer (0.1 M), and the absorbance at 415 nm was determined. If the molar absorptivity is 15,000 L/cm/mol at this wavelength and pH, and the path length is 1 cm, what absorbance reading do you expect?
I understand that we use Beer's Law to find absorbance....but I'm having trouble trying to determine the concentration of PNP so that I can plug it into the equation and solve. Any help would be great. Thanks a lot in advance!!

Explanation / Answer

PNP moles = PNP mass in gm / molar mass = 2.8 x 10^-3/140 = 2 x 10^-4

now vol initially = 20 ml = 0.02 L

Molarity = moles /vol = 2x10^-4/0.02 = 0.001 M

now 0.1 ml of this solution taken and diluted by 4.9 ml i.e final vol = 4.9+0.1 = 5ml

we use formula M1V1 = M2V2 to find final conc after dilution

(0.001 ) x ( 0.1/1000) = M2 x ( 51000)

M2 = 2 x 10^-5 Molar is final conc which is used

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