Chem26 Atqex4.Docx

acCOMMON-ION EFFECT AND BUFFERS

J. BRILLANTES1 and D. ABDAO2

1NATIONAL INSTITUTE OF MOLECULAR BIOLOGY AND BIOTECHNOLOGY

2INSTITUTE OF BIOLOGY

UNIVERSITY OF THE PHILIPPINES, DILIMAN, QUEZON CITY 1101, PHILIPPINES

DATE SUBMITTED: 16 FEBRUARY 2015

DATE PERFORMED: 10 FEBRUARY 2015

ANSWERS TO QUESTIONS

In the experiment, Solution 1 changed to faint red in color after adding methyl orange indicator; according to Table 1 [1] (see Appendix), this indicates a pH less than 3.1 signifying an acidic solution. This event is justified by the partial dissociation of acetic acid in water to form acetate and hydronium ions as in the equation,

CH3COOH + H2O ↔ CH3COO- + H3O+ (1)

which affects the following reaction of methyl orange (ionization of the indicator in water),

HInd + H2O ↔ H3O+ + Ind- (2)

The increase in hydronium ions produced in Equation 1, pushed Equation 2 to the left because of the addition/increase of the product H3O+. This caused the formation of HInd, making it predominant [2]. With this, the solution showed a distinct color, in this case faint pink which Table 1 shows as acidic, because of the predominance of HInd. On the other hand, Solution 2 changed to gold/yellow orange after adding the indicator which Table 1 shows as having pH greater than 4.5 signifying a less acidic solution. This event is caused by the increase of CH3COO- in Equation 1 due to the two sources, acetic acid and sodium acetate; this increase makes the H3O+ to be used up. This decreases hydronium ions in Equation 2, pushing the reaction to the right to relieve the stress [2]. This causes the formation of Ind- making it predominant. Its predominance showed another distinct color, in this case yellow-orange which Table 1 shows to be less acidic. In conclusion, the difference in the colors of Solution 1 and 2 is due to the difference in the concentration of H3O+ that determines whether the indicator equation will be pushed to the left or...