Analytical Chemistry - Spectrophomet Det.

Question

You prepare a standard solution of ferrous ammonium sulfate that is 50.0 ppm in Fe. You pipette aliquots of 0.50 mL, 1.00 mL, 2.00 mL, 4.00 mL, 8.00 mL, and 10.00 mL into 100-mL volumetric flasks, add 1 mL of hydroxylamine solution, 3 mL of o-phenanthroline solution, and sufficient citrate to adjust to pH 3-4 as described in your laboratory instructions. After diluting each sample to the 100 mL mark, and allowing time for the color to develop, you record the absorbances shown in the table.

The data is a bit scattered. Looks like your pipetting ability could use some improvement. However, you can still use this data as a calibration curve. Plot the data in Excel, obtain a trendline and the equation for the linear regression line.

Print the graph with the equation and turn in to your laboratory instructor with your name and section printed at the top.

Fe Conc.(ppm)	Absorbance
0.50	0.157
1.0	0.176
2.0	0.434
3.0	0.582
4.0	0.750
5.0	0.948

Use this graph to answer the following questions:

The equation for the line is in the form "y = mx + b", where m is the slope, and b is the intercept.

I'm not sure if I'm answering these wrong or if my homework software is incorrect. I've had problems with it before.

What is the slope?

What is the Intercept

What is the concentration of Fe in your unknown?

Answer 1

I decided to see if I remember how to use the graphing in Excel, so I did enter the data and plotted it and got the linear regression line.

y = 0.18x + 0.0429

slope = 0.18

intercept = 0.0429

The concentration of Fe in your sample can be calculated using the above equation. You didn't give us the absorbance reading of your unknown, but let's say it is 0.498, then you would do the followig;

0.498 =0.18x + 0.0429

x = 2.53 ppm