Question

Exactly 5.00-mL aliquots of a solution containing phenobarbital were measured into 50.00-mL volumetric flasks and made basic with KOH. The following volumes of a standard solution of phenobarbital containing 2.000 μg/mL of phenobarbital were then introduced into each flask and the mixture was diluted to volume:

0.000, 0.500, 1.00, 1.50, and 2.00 mL. The fluorescence of each of these solutions was measured with a fluorometer, which gave values of 3.26, 4.80, 6.41, 8.02, and 9.56, respectively.

(a) Plot the data.

(b) Using the plot from (a), calculate the concentration of phenobarbital in the unknown

Short answer

The plot of the graph is attached and the concentration of the phenobarbital is 0.410

Step-by-step solution

Given information

5.00-mL aliquots of a solution containing phenobarbital were measured into 50.00-mL volumetric flasks and made basic with KOH.

The fluorescence for respective volumes is provided in the question.

Plotting of graph

From the given data we can plot the graph for absorbance (fluorescence) and concentration (volume).

Calculation of concentration

Extending the straight line obtained in the plot we obtain V_s(initial) as shown in the figure below:

Now, the V_s(initial) value can be used in the equation below to determine the concentration of the unknown,C_x:

$$\begin{gathered} C_x=\frac{-V_s\left(initial\right)C_s}{V_x} \\ {C}_x=\frac{-1.02576\times 2}{5} \\ {C}_x=0.410 \end{gathered}$$