A student did this experiment, and obtained the following results: 1. mass of co

Question

A student did this experiment, and obtained the following results: 1. mass of copper wire: 0.693 g 2. Standard Solutions Solution 1: 0.693 g of wire dissolved in 50 mL solution Solution 2: A 1:2 dilution of solution 1 Solution 3: A 1:4 dilution of solution 1 3. Absorbance data at lambda max Solution/ Absorbance Solution 1 0.810 Solution 2 0.403 Solution 3 0.195 4. Least squares analysis of these data gave the following results: slope =59.10 (units to be found below) y-intercept =0.0080 5. Mass of brass sample: 0.749 g. This sample was dissolved to make 50.00 mL of solution, which was found to have an absorbance of 0.425 at l max . Answer the following questions about this experiment: (a) Calculate the concentrations of the three standard solutions, in units of g/mL. (b) If the concentrations are in units of g/mL, what are the units for the slope? (c) What are the units for the y-intercept? (d) Calculate the percent copper in the brass sample.

Explanation / Answer

a)

0.693 g in 50 mL

Concentration = 0.693 g/ 50 mL

                        = 0.0139 g/mL

1:2 dilution of solution 1:

Concentration will be halved: 0.0139 g/mL / 2 = 0.00695 g/mL

1:4 dilution of solution 1:

Concentration will be 1/4th of solution 1: = 0.0139 g/mL / 4 = 0.00348 g/mL

Concentration Absorbance

0.0139                  0.810

0.00695                  0.403

0.00348                 0.195

Plot a graph between concentration and absorbance. Determine the value of concentration of the brass sample using the given unknown absorbance. From the concentration and volume (50 mL) calculate mass of copper. Then use mass of copper and brass to determine % of copper

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