Question

help! i'm not sure how to go about this.

ABSORBANCE PLOT DATA Please attach a copy of your print out from the spectrophotometer, or indicate that it is attached to your lab partners report and give their name. ron A= emll Analytical wavelength used (=(A-b)lum CALIBRATION CURVE DATA Concentration of Stock Solution: 5.00 X 10 M Door Test Tube Volume Final Volume Final Stock Soln. Solution Concentration 1 2.00 10.0 Absorbance Trial 1 Absorbance Trial 2 2. 4.00 10.0 3 6.00 10.0 0.0670 0.128 0.193 0.256 0.320 4 8.00 10.0 5 Stock Solution 5.00 x 100 6 0 Tap Water 7 Unknown #1 0.001 0.317 7 Use Microsoft Excel to plot your data and determine the unknown concentration. To receive credit, you must turn in your Excel plot. Your Excel plot must include a title, labeled x and y-axis, and the equation for the line. Average final concentration of unknown absorban concentration POST-LABORATORY QUESTIONS

Answer 1

Calibration curve data:

Concentration of stock = 5.00 x 10^-5 M

Final Concentration = Concentration of stock * Volume of stock/Final volume

Final volume for each tube = 10.00 mL

For tube 1, Final Concentration = 5.00 x 10^-5 M * 2.00 mL/ 10.00 mL

= 1.00 x 10^-5 M

For tube 2, Final Concentration = 5.00 x 10^-5 M * 4.00 mL/ 10.00 mL

= 2.00 x 10^-5 M

For tube 1, Final Concentration = 5.00 x 10^-5 M * 6.00 mL/ 10.00 mL

= 3.00 x 10^-5 M

For tube 1, Final Concentration = 5.00 x 10^-5 M * 8.00 mL/ 10.00 mL

= 4.00 x 10^-5 M

Volume of stock is different in each testube.

Testube	Volume of stock solution	Final volume	Final concentration	Absorbance Trial 1
1	2.00	10.00	1.00 x 10-5 M	0.067
2	4.00	10.00	2.00 x 10-5 M	0.128
3	6.00	10.00	3.00 x 10-5 M	0.193
4	8.00	10.00	4.00 x 10-5 M	0.256
5	Stock solution	5.00 x 10-5 M	0.320
6	Tap water	0	0.007
7	Unknown	---	0.317

The calibration curve can be drawn as :

0.35 Standard curve of stock solution 0.3 y = 0.0628x +0.0049 0.25 0.2 Absorbance 0.15 0.1 0.05 0 0 1 2 3 4 5 6 Concentration (105M)

From the graph, equation of straight line is y = 0.0628x + 0.0049

Absorbance of unknown = 0.317 M

Calculate the concentration of unknown as shown below:

0.317 = 0.0628x + 0.0049

x = 4.97 x 10^-5 M

Hence, concentration of unknown is 4.97 x 10^-5 M

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Post Lab Questions

Answer 1a) Correct choice is option a)

Reason: It is because increasing the concentration of stock will increase the concentrations of all the standard solutions.

Answer 1b) Correct choice is option a)

Reason: It is because absorbance is directly proportional to concentration. Increasing the concentration of stock will increase the concentrations of all the standard solutions and hence, the absorbance becomes higher.

Answer 1c) Correct choice is option c)

Reason: It is because slope is the ratio of absorbance to that of concentration. As both these increase to same extent, the slope will remain unchanged.  

Answer 1d) Correct choice is option c)

Reason: It is because, absorbance of unknown is independent of absorbance of the stock and will remain same as before.

Answer 2

Use of 20.0 mL volumetric flask will increase the volume of solution which will inturn decrease the concentration of all the standards.

So, the answers are:

Answer 2a) Correct choice is option b)

Reason: It is because concentration inversely varies with volume. So, increase in volume decreases the concentrations of all the standard solutions.

Answer 2b) Correct choice is option b)

Reason: It is because absorbance is directly proportional to concentration. Decrease in volume decreases the concentrations of all the standard solutions and hence, the absorbance becomes lower.

Answer 2c) Correct choice is option c)

Reason: It is because slope of line is the ratio of absorbance to that of concentration. As both these decrease to same extent, the slope will remain unchanged.  

Answer 2d) Correct choice is option c)

Reason: It is because, absorbance of unknown is independent of absorbance of the stock and will remain same as before.

Answer 3

The effect of concentration on absorbance is as per Lambert Beer law. The concentration directly affects absorbance.

Two points always form a straight line while three or more points will affect the slope of straight line curve.

Five standard solution will give five different points which will determine the overall slope of the calibration curve. This will decide the proportionality constant between the concentration and absorbance which is not possible with two points.

Answer 4

If concentration of stock is less than the given value, the concentration of each of the standard solutions will decrease. Absorbance being directly affected by concentration also decreases with this.

Due to equivalent amount of decrease in concentration as well as absorbance, the slope of the standard curve will remain same and hence, the equation of straight line will also be same.

Concentration of unknown is calculated from the equation of straight line and will be same irrespective of the concentration of the stock solution.

Hence, there will be no effect on the concentration of unknown.