Question

Determination of Phosphate in Water Out-of-Class Exercise 4. Use Data Set I t o answer the following questions a) Make a plot of absorbance versus concentration for the data set. Draw the best straight line or smooth curve (whichever is appropriate). Put the absorbance on the yaxis and concentration on the cas. b) Make a plot of % transmittance versus concentration for the data set. Draw the best straight line or smooth curve (whichever is appropriate). Put the transmittance on the y-axis and concentration on the .c-aris. c) Which of the plots (absorbance vs. concentration or Transmittance vs. concentration) would probably be best to use to determine the concentration of other solutions of the same species? Why? ? d) What would be the concentration of a solution that has an absorbance of 0.M Be sure to show your work on the graph. M solution? Be sure to e) What would be the value of the absorbance of a 2.UNIO show your work on the graph.

Answer 1

See the graph below drawn using Microsoft excel.

Note that the concentration has been in units of (10^-4 M)

a) Absorbance vs Concentration plot

Note that here a near straight line is a better fit

b) % Transmittance vs Concentration plot

Here, an exponential decay is observed.

х absorbance plot.xlsx - Microsoft Excel Insert Page Layout Formulas Data Review Chart Tools Design Home View Layout Format al 7 === م م م م م م م م Change Save As Switch Select Chart Type Template Row/Column Data Type Data Move Chart Location Chart Layouts Chart Styles Chart 8 fic A G H M N O P Q Absorbance 0.16 0.22 1.9 % Transmittance Concentration in 10^-4 M 69.2 5.8 60.5 33.9 23.2 13.9 11.5 0.47 0.63 0.86 0.94 % Transmittance TI Ut % Transmittance Absorbance TIL Absorbance Linear (Absorbance) — 1 10 20 30 40 LLLLL 0 10 40 19 20 21 22 23 ™ Ready 20 30 Concentration K Sheet1 Sheet2 Sheet3 QX* DO U 100% ? ) @ 30 + ENG 1:36 PM US 20-Nov-19 9

c) The Absorbance plot would be best. It is because, The Absorbance vs concentration plot follows a linear relationship

This is also true as per the Lambert Beer Law which states that Absorbance is proportional to concentration of the specie.

d) Now, For Absorbance of 0.19, see the orange colored line (in extra bold for explanation). The concentration on x- axis comes at about 7 x 10^-4 M
e) For concentration of 2.4 x 10^-3 M or 24 x 10^-4 M, The absorbance is around 0.65. see the greencolored line (in extra bold for explanation)

absorbance plot.xlsx - Microsoft Excel х Home Insert Page Layout Formulas Data Review View * Cut En Copy Paste Format Painter Clipboard Calibri B I U 27 - 11 E A - A^ A = = = 2 E Wrap Text B Merge & Center - - : General $ % O Insert Delete Format Autosum cleare 1 D Conditional Format as Formatting Table Styles , Cell Styles 2 Clear Sort & Filter Editing Find & Select Font Alignment Number Cells 122 A A G H I J N O P Q B 23.2 13.9 11.5 D 0.63 0.86 0.94 K L M % Transmittance % Transmittance Absorbance Absorbance Linear (Absorbance) — 1 10 20 30 40 0 10 30 40 20 Concentration 23 24 25 26 27 ™ Ready K Sheet1 Sheet2 Sheet3 QX* DO U 100% ? ) @ 30 + ENG 1:55 PM US 20-Nov-19 9