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CHEM 146
Lab 2 Worksheet |
1
Lab 2 Worksheet
GRADE ____ /25 p
Your Name:
Instructions. Carefully review the
Constant-Volume Standard Addition material (pages 7 – 13) of the
CHEM 146 Lab 2 Manual given its central importance (especially the fully worked example on pages 11 – 13). In this online activity, you are processing and analyzing raw FAAS data identical to what you would obtain in lab.
FAAS Lead Analysis by Constant-Volume Standard Addition. Assume we wish to find the Lead (Pb) concentration in ppb for a lake water sample (the “unknown”). Using a pipette, 10.00 mL of lake water is loaded into five 50 mL volumetric flasks. Next, exactly 0.00, 5.00, 10.00, 15.00 and 20.00 mL of standard lead solution at 4.96 ppb (parts per billion) is added. After dilution to the volume, FAAS readings at = 283 nm are taken of the five solutions. The resulting absorbance values are presented below.
|
Table 2-1. FAAS Lake Water Sample Lead Analysis. FAAS Absorbance Readings for Lead Standard Volume Additions |
|
|
(mL) |
Absorbance at 283 nm; |
|
0.00 |
0.194 |
|
5.00 |
0.349 |
|
10.00 |
0.485 |
|
15.00 |
0.645 |
|
20.00 |
0.789 |
1. Standard Addition vs. Plot for the Lake Water Sample. Plot in EXCEL the vs. data. You may find pages 1 – 7 of the
Lab 2 Supplement – EXCEL Graphing & LINEST document on D2L particularly helpful. Include the
R2 value and the equation of the linear trend-line (both from EXCEL). You will likely need to adjust the sig fig display for the
R2 value and linear trend-line equation in EXCEL. Properly title the plot and label the axes correctly. Use reasonable sig figs and correct units. Paste in the complete plot below.
2.
EXCEL LINEST Data Workup. Run LINEST (in EXCEL) on the
Table 2-1 data set to obtain
m,
sm,
R2,
b283 and
sb for the lead determination. You should find the information on pages 7 – 10 of the
Lab 2 Supplement – EXCEL Graphing & LINEST document on D2L very helpful. [If you cannot run LINEST, see the “work around” on page 10 of the supplement for
m,
sm and
R2. The
y-intercept,
b283, can be obtained from the linear trend-line equation of the vs. graph. Ask your lab instructor for
sb.] Submit your complete EXCEL LINEST or “work around” file to the D2L Lab 2 Worksheet DropBox along with this document. Present the LINEST (or “work around”) results (
m,
sm,
R2,
b283 and
sb) for the lead determination to at least four significant figures below.
FAAS Constant-Volume Standard Addition Lead Analysis LINEST (or “Work around)” Results
·
m:
·
sm:
·
R2:
·
b283:
·
sb:
3. Unknown (Lake Water) Lead Concentration. Using the analysis values above, calculate the lead ppb via,
Show all work by presenting your calculation in a clear, organized manner. First state symbolically the equation used, then present the calculation in which actual values are substituted in and the results shown.
Remember this format for future calculations. It is recommended to carry the calculation to four significant figures to avoid rounding errors and round to the correct SF at the end.
4. Unknown (Lake Water) Lead Analysis Sample Standard Deviation. Calculate the sample standard deviation (
sc) for the standard addition lead analysis using,
Show all work by presenting your calculations in a clear, organized manner. Remember to first state symbolically the equation, then present the calculation with actual values substituted in and the result shown. It is recommended to carry the calculation to four significant figures to avoid rounding errors and only round to the correct SF at the end.
5. Unknown (Lake Water) Lead Analysis Mean and Sample Standard Deviation. Write the lead analysis results in the form for the lake water sample in the space below. (For your reference, the EPA action threshold for lead in public drinking water systems is 15 ppb, or 15 g/L.)
146 Lab 2 Worksheet
v1.2
MY, SG © 2021
7-3-21
UA-CBC Technical Press, University of Arizona
