A ground water sample was analyzed for toluene by the standard additions method. Five 4.00-mL aliquots of the sample were placed in five separate 50.0-mL volumetric flasks. The first flask was set aside and exactly 2.00 mL, 4.00 mL, 6.00 mL, and 8.00 mL of a 10.25 ppm toluene standard was added to each of the other flasks, respectively. All flasks were diluted to volume with distilled water. The absorbance of each solution was measured by UV-Vis spectroscopy and the following data was obtained.

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The chromium in an aqueous sample was determined by pipetting 5.0 mL of the unknown into each of five 50.0-mL volumetric flasks. Various volumes of a standard containing 12.8 ppm Cr were added to the flasks, following which the solutions were diluted to volume.

Unknown, mL	Standard, mL	Absorbance
5.0	0.0	0.201
5.0	10.0	0.292
5.0	20.0	0.378
5.0	30.0	0.467
5.0	40.0	0.554

Plot the data using a spreadsheet and determine the concentration of Cr in ppm in the sample. (You need to enter only this number with three significant figures.)

The data below are for analysis of an unknown solution by the method of standard additions. 5.00 mL of unknown solution are added to 10-mL volumetric flasks, along with zero to 4 mL of a 14.4 mg/L standard solution. Then DI water is added to bring the volume up to 10.00 mL, the flasks are mixed well, and their absorbances are measured. A blank sample gave an absorbance of zero.

Solution	mL of Unknown Solution	mL of 14.4 mg/L Standard Solution	Total Volume, mL	Absorbance
0	5.00	n/a	10.00	0.137
1	5.00	1.00	10.00	0.296
2	5.00	2.00	10.00	0.404
3	5.00	3.00	10.00	0.563
4	5.00	4.00	10.00	0.672

(a) Use only solutions 0 and 1 with the single-spike method of standard additions to calculate the concentration of the unknown. (Remember, this is asking for the concentration of the original unknown solution so you will have to account for the fact that 5.00 mL of unknown was diluted to 10.00 mL before absorbance was measured.)

mg/L

(b) Use all five solutions and the multi-spike (linear regression) method of standard additions to calculate the concentration of the unknown.

mg/L

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