Pre-Lab Questions
Data:
[mass of copper wire]
0.738 g
[standard solutions]
solution 1: 0.738 g of copper wire dissolved in 50.00 mL solution
solution 2: 1:2 dilution of solution 1
solution 3: 1:4 dilution of solution 1
[absorbance data at lambda(max)]
solution 1: 0.740
solution 2: 0.365
solution 3: 0.195
[least squares analysis of these data pieces]
slope: 49.458
y-intercept: 0.0075
A brass sample of mass 0.643 g was dissolved to make 50.00 mL of solution, which was found to have an absorbance of 0.589 at lambda(max).
Questions:
1) Calculate the concentrations of the three standard solutions (in g/mL).
solution 1: 0.738 g / 50.00 mL = 0.0148 g/mL
solution 2: 0.0148 g/mL / 2 = 0.740 g/mL
solution 3: 0.740 g/mL / 2 = 0.370 g/mL
2) If the concentrations are in units of g/mL, what are the units for the slope?
If absorbance (A) is graphed against concentration (C) then, using the least squares method, the best-fit straight line through the data will be:
A = mC + b
m (slope) = delta y / delta x = delta A / delta C
A is dimensionless. C has units of g/mL. Therefore, the slope for the data presented above is 49.458 (g/mL)^(-1).
3) What are the units for the y-intercept?
The y-intercept (b) is found when concentration is 0. This yield only a numerical value for A without any units.
Therefore, the y-intercept for the data presented above is already correct as 0.0075.
4) Calculate the percent copper in the brass sample.
This is where I'm stuck. I have no idea how to use the absorbances of the various solutions to find the percent copper in the brass sample.
The only step I'm fairly certain about is:
% copper in brass = (mass of copper in brass sample) / (mass of brass sample) *100%
% copper in brass = (mass of copper in brass sample) / (0.643 g) *100%
Any feedback regarding my work for the first 3 questions and any help on the 4th question is greatly appreciated. Thanks in advance.
I assume you have drawn a graph to find the straight line. I also assume you have measured the absorbance of one or more of the brass samples. Using A from the unknown sample, look at the graph and convert A to C. C will be in g/mL. Use that to convert g/mL to grams of the sample, then calculate % Cu in the usual manner. There is insufficient data and/or explanation to comment on the other parts.
For the first three questions, your calculations are correct. Here's a summary of the answers:
1) Concentrations of the three standard solutions (in g/mL):
- Solution 1: 0.0148 g/mL
- Solution 2: 0.740 g/mL
- Solution 3: 0.370 g/mL
2) The units for the slope are (g/mL)^(-1). This means that for each unit increase in concentration (g/mL), the absorbance decreases by the slope value (49.458 g/mL)^(-1).
3) The y-intercept has no units since it represents the value of the absorbance when the concentration is 0.
Now, let's move on to question 4.
To calculate the percent copper in the brass sample, we need to use the absorbance value and the calibration curve equation:
A = mC + b
We know the absorbance of the brass solution (A = 0.589), and we can rearrange the equation to solve for the concentration (C):
C = (A - b) / m
Substituting the values from the given data:
C = (0.589 - 0.0075) / 49.458 g/mL^(-1)
C = 0.5815 / 49.458 g/mL^(-1)
C ≈ 0.011752 g/mL
To calculate the mass of copper in the brass solution, we can use the concentration and the volume of the solution:
Mass of copper in brass sample = concentration × volume
Mass of copper in brass sample = 0.011752 g/mL × 50.00 mL
Mass of copper in brass sample = 0.5876 g
Finally, we can use this mass of copper and the mass of the brass sample to calculate the percent copper:
% copper in brass = (mass of copper in brass sample) / (mass of brass sample) × 100%
% copper in brass = 0.5876 g / 0.643 g × 100%
% copper in brass = 91.24%
Therefore, the percent copper in the brass sample is approximately 91.24%.