Question

Data: Consider the data below for a Beer's Law calibration curve for an aqueous orange dye solution that has 2. mx450 nm. The solutions used were translucent. The data was collected using a 1.20cm wide test tube cuvette, using 2. = 450 nm. Trial Concentration (M) Absorbance P ercent Transmittance 1 0.0030 0.153 70.3 2 0 .0060 0.312 48.8 3 0 .0090 0.465 34.3 0.0120 0.627 23.6 0.0150 0.771 16.9 For each of the following errors/changes, indicate whether a repeat of the experiment with only that change would produce absorbance readings that would: a) be lower, b) stay the same, c) be higher, or d ) frequently be lower and frequently be higher than the data above. (pick one letter a, b, c, or d for each change) The cuvette is changed to a 2.54cm wide cuvette. The wavelength is changed to 550 nm. 3. The experiment is repeated, but with a different 1.20cm cuvette. This other cuvette contained an unreactive impurity that makes the solution cloudy.

Answer 1

Beer Lambert Law:

The law states the absorbance of a solution is directly proportional to the concentration of a solution and the path length.………………………………………..(1)

А— sC!

Where A = Absorbance of solution

ε = constant (epsilon) or molar absorptivity constant

C = concentration

l = path length

From the data given in the question it can be seen clearly that with increase in concentration the absorbance increases.

Q 1. If we change the width of cuvette to 2.4 cm from 1.25 cm:

Ans: On increasing the width of the cuvette the path length will also increase. It is clearly evident from the equation 1 that absorbance is directly proportional to path length. Hence on increasing the path length the absorbance will increase for every observation. (Option c – higher)

Q 2 Effect of change of wavelength from 450 nm to 550 nm.

Ans: On changing the wavelength the molar absorptivity constant changes. The increase in wavelength may result in increase or decrease of absorbance values at a given ε. (Option d – frequently be lower or frequently be higher than the data)

Q 3 Effect of using an impure cuvette

Ans: If the solution is cloudy then it will become difficult for the light to pass through and hence less light will pass through the solution. The solution has become turbid now. So turbidity is calculated as Ln (It/Io) and absorbance is calculated as Log (It/Io). Increasing the turbidity will increase the absorbance as A = Turbidity x path length / 2.3. Hence Absorbance will be higher (Option c – higher)

Beer Lambert Law:

The law states the absorbance of a solution is directly proportional to the concentration of a solution and the path length.

………………………………………..(1)

А— sC!

Where A = Absorbance of solution

ε = constant (epsilon) or molar absorptivity constant

C = concentration

l = path length

From the data given in the question, it can be seen clearly that with the increase in concentration the absorbance increases.

Q 1. If we change the width of the cuvette to 2.4 cm from 1.25 cm:

Ans: On increasing the width of the cuvette the path length will also increase. It is clearly evident from equation 1 that absorbance is directly proportional to path length. Hence on increasing the path length, the absorbance will increase for every observation. (Option c – higher)

Q 2 Effect of change of wavelength from 450 nm to 550 nm.

Ans: On changing the wavelength the molar absorptivity constant changes. The increase in wavelength may result in increase or decrease of absorbance values at a given ε. (Option d – frequently be lower or frequently be higher than the data)

Q 3 Effect of using an impure cuvette

Ans: If the solution is cloudy then it will become difficult for the light to pass through and hence less light will pass through the solution. The solution has become turbid now. So turbidity is calculated as Ln (It/Io) and absorbance is calculated as Log (It/Io). Increasing the turbidity will increase the absorbance as A = Turbidity x path length / 2.3. Hence Absorbance will be higher (Option c – higher)