Determination of Aspirin by Spectrophotometric Analysis 1. One way to judge the

Question

Determination of Aspirin by Spectrophotometric Analysis 1. One way to judge the quality of the results is to look at the calibration plot. The closer all the points are to the line, then the "better" the plots and results. A plot with a straight line with very few points on the line or close to the line has a lot of uncertainty in it. Looking at your plot and your results, how contident are you of your results? Explain your level of confidence in your plot and results 2. One tablet of Brand Y aspirin was dissolved in 10 mL. of 1.0 M NaOH, boiled, and then diluted to a final volume of 100 mL with deionized water. A 2.5 ml. aliquot of this concentrated aspirin solution was diluted to a final volume of 100 mL with FeCl, A calibration plot of absorbance vs. concentration was obtained, with the slope of the best-fit straight line as 1370 M-.The diluted aspirin solution had an absorbance of 0.932. What is the concentration of the dilute aspirin? a. What is the concentration of the aspirin in the dilute aspirin solution? b. What is the concentration of the original aspirin solution? c. How many mg of aspirin are present in one tablet of Brand Y aspirin?

Explanation / Answer

a)The Callibration plot is between Absorbance Vs concentration of Standard Solutions of Known concentrations.

It is based on the Equation, A=eCl (Beer's law),

where A=absorbnce

e=absorptivity of the species

l=path length

Slope=el (As it is a linear plot of the form y=mx+c with c=intercept=0 an m=slope=el)

Given slope=el=1370 M^-1

Absorbance of dil Aspirin=0.932

So,C=concentration of diluted aspirin=A/slope=0.932/1370 M^-1=6.803*10^-4M

b)Cdil*Vdil=Cconc*Vconc

Cdil=concentration of diluted aspirin=6.803*10^-4M

Cconc=concentration of original aspirin

Vdil=volume of diluted aspirin=100 ml

Vconc=concentration of original aspirin=2.5ml

Cconc=Cdil*Vdil/Vconc=(6.803*10^-4M)*100ml/2.5ml=0.0272M

c)

FeCl3 test of aspirin

FeCl3+Aspirin --->(CH3CO) Ph-OFeCl2 (Fe-aspirin complex that gives the color]

So .the complex molecule has 1 mol asprin:1 mol FeCl3

Cconc=concentration of original aspirin=0.0272 mol/L=0.0272 mol/L *molar mass=0.0272 mol/L*180.157 g/mol=1.51*10^-4 g/L=1.51*10^-4 g/L* 1000 mg/g=0.151 mg/L

volume of original solution prepared=100 ml=0.1L

concentration of original aspirin in100ml=0.1L*0.151 mg/L=0.0151 mg

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