Logarithmic Trends Not all relationships are perfectly linear, many data sets found in science have exponential or logarithmic relationships. For example, in the study of spectroscopy, light is shined through a solution, and the amount of light that passes through to the other side is measured. The amount of light transmitted (coming out the other side) depends on the chemical and the concentration. As the concentration of a solution increases, the amount of light passing through it decreases, and does so in a logarithmic way. This data can be difficult to interpret, however by accounting for the log function, a linear trend is seen that can be easily correlated to the concentration. Transmittance is the fraction of light passing through divided by the total amount of light. The amount of light that is blocked by the sample (Absorbance) can be determined the equation
= − (%)
Plot the data below from a sample of Copper (II) Sulfate solution.
Prepare two graphs, the first graph should plot the Concentration on the X-Axis vs the % transmittance on the Y-Axis.
Convert the Transmittance to Absorbance using the above equation and prepare a second graph plotting Concentration vs. Absorbance.
Graph using excel.
Concentration %Transmittance Absorbance
0 mm 100%
10 mm 80%
20 mm 64%
40 mm 41%
60 mm 26%
80 mm 17%
100 mm 11%
120 mm 7%
140 mm 5%
160 mm 3%
180 mm 2%
200 mm 1%
Using the data provided, plot the concentration on the X-Axis vs the %Transmittance on the Y- Axis. Include this graph in your report.
1. What is the slope of the second graph?
2. What is your unknown number?
3. What is your unknown concentration of Copper (11) Suflate?
4. What would be the Absorbance?
5. What would be the % Transmittance?
6. What is your unknown %Transmittance of Copper (11) Suflate?
7. What is the concentration?