11) Two different 1.2 L buffered solutions were prepared using HOBI and LiOBr. B
ID: 1027013 • Letter: 1
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11) Two different 1.2 L buffered solutions were prepared using HOBI and LiOBr. Both buffered solutions had a pH of 5.2 at 25°C. After 0.17 moles of HI were added to each of the solutions, it was found that the pH of one solution had dropped to 4,9 and the pH of the other had dropped to 3.1. 8. What is the balanced net ionic equation for the reaction that occurred when HI was added to these buffered solutions? b. What trend must be true when comparing the concentrations of HOBr and OBrin the two solutions if they shared the same pH before the HI was added? Justify your answer. (Hint: The concentrations of acid in the solutions are not the same.) c. Explain why the pH of the two solutions ended up being different after the same amount of HI was added to each.Explanation / Answer
a) The reactions in the buffer systems can be represented as
HOBr (aq) --------> H+ (aq) + OBr- (aq)
OBr- (aq) + H2O (l) --------> HOBr (aq) + OH- (aq)
Buffer 2 consists of LiOBr. Li+ is the spectator ion and OBr- is the conjugate base of the weak acid HOBr which reacts with the water (H2O) in the aqueous solution to give HOBr and generates OH-.
HI is a strong acid and ionizes completely in aqueous solutions to produce H+ and I-. The net ionic equations for the reactions of HI with the two buffers are shown below.
HOBr (aq) + HI (aq) --------> 2 H+ (aq) + OBr- (aq) + I- (aq) …..(1)
OBr- (aq) + HI (aq) ---------> HOBr (aq) + I- (aq) …….(2)
b) The pH of both buffers 1 and 2 were the same before the addition of HI. After the addition of HI, the pH of one buffer (say buffer 1) dropped more than that of the other buffer (say buffer 2). The lower the pH of a solution, the more acidic is the solution. Consequently, buffer 1 must have a higher concentration of the weak acid (HOBr) than buffer 2. Therefore, buffer 2 has a higher concentration of LiOBr as compared to buffer 1.
c) The same amount of HI was added to both the buffers. However, it has been already stated that the pH of buffer 1 dropped more than the pH of buffer 2. This can explained on the basis of reactions and 2 above. Since buffer 1 had a higher concentration of HOBr (more H+), adding HI increased the concentration of H+ and lowered the pH of the buffer significantly (to 3.1). Buffer 2 had a higher concentration of LiOBr (OBr-) and some of the added HI was used to neutralized OBr- to HOBr. As stated, HOBr is poorly ionized. Hence, even though HOBr was formed in the second buffer, as the initial concentration of HOBr was low, the H+ concentration does not increase significantly and hence, the pH drop was lower.
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