Aspirin Lab Report

Decomposition of Aspirin Studied with UV/Visible Absorption Spectroscopy

Aims:
 To determine the concentration of salicylic acid, formed from the hydrolysis of Aspirin, at regular intervals using the UV/Visible Absorption Spectroscopy
 From the concentration of salicylic acid, concentration of Aspirin to be determined using an equation
 Calculate the rate constant of this reaction and its order from a plot of graph of ln(aspirin) vs time
 Discuss the overall flaws and improvements to the experiment

Results:

As per schedule1, 0.212g of aspirin was added to 50 ml boiling water to form salicylic acid in a 100 ml flask, of which 1 ml was then pipetted to a 50 ml volumetric flask at the 5th min. Following an ice bath, the solution was mixed

Upon finding the actual concentrations of salicylic acid, concentration of aspirin in the flask at various times can be found using the equation [aspirin]t = [aspirin]0 – [salicylic acid], since at constant volume, number of moles of initial aspirin decrease to form salicylic acid.
Initial concentration of aspirin formed as follows: [aspirin]0 = 0.212g / (180.157gmol-1 * 50/1000 L) = 0.0235 mol L-1. Thus using the first test as sample, [aspirin]t = 0.0235 – 9.981*10-4 = 0.0225 mol L-1. To find the rate constant, we will need to log the value of [aspirin]t and plot it against time to find the rate constant.

Figure 1 shows the diluted and actual concentrations of salicylic acid, the concentration and log value of aspirin at various times.

Time / min C1 / mol L-1 C2 (actual) / mol L-1 [Aspirin] / mol L-1 ln([Aspirin])
0 1.996*10-5 9.98*10-4 0.0225 -3.79
10th 6.925*10-5 3.46*10-3 0.0200 -3.91
20th 1.135*10-4 5.68*10-3 0.0178 -4.03
30th 1.372*10-4 6.86*10-3 0.0166 -4.10
40th 1.653*10-4 8.27*10-3 0.0152 -4.18
50th 1.828*10-4 9.14*10-3 0.0144 -4.24
60th 1.953*10-4 9.77*10-3 0.0137 -4.29
Figure 1.

A graph of ln([aspirin]t) against time (min) was plotted. The gradient gave the value of K, the rate constant for the reaction.

Figure 2 shows the plotted graph

Figure 2.

From the

The hydrolysis formed salicylic concentration which was mixed with iron(III) solution to form a purple complex. This was then use to study under the UV/Visible absorption spectroscopy which gave absorbance values recorded at 525nm to determine the concentration of salicylic acid using the Beer Lambert’s Law and later corrected to find the actual concentrations. The concentrations of aspirin at various intervals was found from salicylic concentrations. Upon plotting a graph of ln(aspirin) vs time, it produced a linear equation from which the gradient gave the rate constant of 0.0083min-1 and the overall shape of the graph concluding this reaction to be pseudo first order with respect to the concentration of aspirin with the deviations and improvements as