Question 3 of 6 Incorrect Incorrect Incorrect Sapling Learning Map Atomic absorp

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Question 3 of 6 Incorrect Incorrect Incorrect Sapling Learning Map Atomic absorption spectrometry with the method of standard additions to determine the concentration of was used mL an waste For the addition, a 1000.0 Cd standard was added to 10.0 mL of solution. The following data were obtained: sorbance of reagent blank -0.039 Absorbance of sample 0.400 Absorbance of sample plus addition 0.782 What was the concentration of the cadmium in the waste stream sample? Number 0.11 ug/ mL Later, the analyst learned that the blank was not truly a reagent blank, but water. The absorbance of the actual reagent blank, was 0.086. Calculate the cadmium concentration using the new information of the Number 0.451 ug/ mL Calculate the percent error caused by using water instead of the reagent blank. Number 3.4 A Previous Try Again 0 Next Exit Explanation

Explanation / Answer

Ans. 1. #1. Absorbance of reagent blank = 0.039

Absorbance of sample = 0.400

            Actual absorbance of sample = Abs of sample – Abs of blank = 0.400 – 0.039 = 0.361

#2. Absorbance of sample + addition = 0.782

            Actual absorbance = 0.782 – 0.039 = 0.743

            Increase in absorbance = Abs of (sample + Addition) – Abs of sample alone

                                                = 0.743 – 0.361

= 0.382

Now,

10.0 µL of 1000.0 µg/mL sample contains 10.0 µg Cd.

            That is, 10.0 µg (= 10.0 10.0 µL) Cd was added to the sample.

            Final volume of the sample + Addition = 10.0 mL

            That is 10. 10.0 µg Cd was diluted upto 10.0 mL

So, [Cd] from addition = mass of CD added / Final volume of solution

                                    = 10.0 µg / 10.0 mL

                                    = 1.0 µg/ mL

Therefore, the final concentration of Cd from addition µg/ mL. Or, the Cd concentration of the solution has increased by 1.0 µg/ mL due to addition.

Note that the increase in absorbance of the sample is due to increase in Cd concentration by 1.0 µg/ mL.

            So, an absorbance of 0.382 is equivalent to 1.0 µg/ mL

                        Or,        -           1.0        -               (1.0 / 0.382) µg/ mL   

            Or,        -           0.361   -               (1.0 / 0.382) x 0.361 µg/ mL

                                                            = 0.9450 µg/ mL

Hence, [Cd] of sample = 0.9450 µg/ mL

Part B: Actual Abs sample = 0.400 – 0.086 = 0.314

            Actual abs (sample + Addition) = 0.782 – 0.086 = 0.696

            Increase in absorbance = 0.696 – 0.314 = 0.382

So, an absorbance of 0.382 is equivalent to 1.0 µg/ mL

                        Or,        -           1.0        -               (1.0 / 0.382) µg/ mL   

            Or,        -           0.314   -               (1.0 / 0.382) x 0.314 µg/ mL

                                                            = 0.8292 µg/ mL

So, modified [Cd] of sample = 0.8292 µg/ mL

Ans. Part C. error = [Cd] actual (part B) – [Cd] incorrect (part A)

= 0.8292 µg/ mL - 0.9450 µg/ mL

= (-0.1158) µg/ mL

% error = (error / Actual [CD]) x 100

                        = { [(-0.1158) µg/ mL] / 0.8292 µg/ mL } x 100

                        = - 13.96 %

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