Chemical Equilibrium of a Weak Acid

542 words | 2 page(s)

In this experiment, the chemical equilibrium of methyl red which is a weak acid is investigated by finding the dissociation constant. Spectrophotometry is used in each case to measure the absorbance for different concentration of the test solutions. The extinction coefficients was obtained as the gradients of the concentration and absorbance graphs, at the wavelengths where the maximum absorption of 〖MR〗^- and HMR to the light occurred. The extinction coefficient therefore aids in obtaining the concentration of deprotonated and protonated methyl red, 〖MR〗^- and HMR, respectively which in turn leads to the finding of 〖pK〗_c and K_c

Materials and Methods
The experiment was carried in accordance to the JCP221-Experiment #3 (Experimental part in pg.3-3) in the laboratory manual. The procedure was carried out as expected apart from a few steps which went off from the procedure. The solutions transfer during the absorption measurement was done using a graduated pipette rather than a Pasteur pipette. The preparation of the fourth solution was also done differently where the solution ended up being prepared using 5 ml of 0.02M NaOAc and 50ml of 0.2M Acetic acid. Apart from this steps, all the other parts went as expected.

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Results and Discussion
According to theoretical sources, the wavelengths at which the maximum absorption of 〖MR〗^- and HMR occurred should be at 430nm and 520nm. In the obtained graphs however, the maximum absorption of 〖MR〗^- and HMR seemed to occur at about 426nm and 525nm respectively which is close to the experimental values. The calculation of the concentration and the respective error analysis is given in the appendix 1. The values of graphs of concentration against absorbance are then given in the appendix 2. The gradient of the graphs gives the extinction coefficient of 〖MR〗^- and HMR are given for the 430nm and 520nm wavelengths at Table 2. Appendix 3 then gives the equations used to obtain the dissociation constants by previously obtaining the concentrations of HMR and 〖MR〗^-.

From Table 2, which shows the extinction coefficient of 〖MR〗^- and HMR, its notable that extinction coefficient of HMR is lesser than that of 〖MR〗^- at 420nm, the wavelength at which 〖MR〗^- acquires maximum absorbance. Also, from the Table 3 a trend is observed where the concentration of HMR decreased with the increase in the volume of the acetic acid to the solution of 10ml MR- and 5ml NaOAc. Similarly, the concentration of 〖MR〗^- increases with the increase in the volume of the acetic acid.

The dissociation constant and pK_c also show a trend in the variation of the volume of the acetic acid. Increasing the volume of the acetic acid causes an increase in the dissociation constant. Similarly, increasing the acetic acid causes a decrease in the value of the pK_c. However, the first instance where the volume of the acetic acid is 5ml shows an error where the values do not correspond to the trend. The error can be attributed either to observations errors or human errors during the conduction of the experiment.

  • Kenkel, John. Basic Chemistry Concepts And Exercises. 1st ed., Boca Raton, Fla, CRC, 2011.
  • Methyl Red sodium salt | C15H14N3NaO2 – PubChem. (n.d.). Retrieved March 01, 2017, from https://pubchem.ncbi.nlm.nih.gov/

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