General Chemistry II - Assignment 4

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[ Print View ] General Chemistry II Assignment 4 Due at 11:00am on Wednesday, August 6, 2008 View Grading Details Adding a Strong Acid to a Buffer Learning Goal: To understand how buffers use their reserves of conjugate acid and conjugate base to ameliorate the effects of acid or base addition on pH. A buffer is a mixture of a conjugate acid-base pair. In other words, it is a solution that contains a weak acid and its conjugate base, or a weak base and its conjugate acid. For example, an acetic acid buffer consists of equimolar amounts of acetic acid, , and its conjugate base, . Because ions cannot simply be added to a solution, the conjugate base is added in a salt form (e.g., ). Buffers work because the conjugate acid-base pair work together to neutralize the addition of or ions. Thus, for example, if ions are added to the acetate buffer described above, they will be largely removed from solution by the reaction of with the conjugate base: Similarly, any added ions will be neutralized by a reaction with the conjugate acid: This buffer system is described by the Henderson-Hasselbalch equation
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Part A A beaker with 165 of an acetic acid buffer with a pH of 5.00 is sitting on a benchtop. The total molarity of acid and conjugate base in this buffer is 0.1 . A student adds 4.80 of a 0.380 solution to the beaker. How much will the pH change? The of acetic acid is 4.76. Hint A.1 How to approach the problem Scott Dukowitz Adding a Strong Acid to a Buffer Part A A beaker with 165 of an acetic acid buffer with a pH of 5.00 is sitting on a benchtop. The total molarity of acid and conjugate base in this buffer is 0.1 . A student adds 4.80 of a 0.380 solution to the beaker. How much will the pH change? The of acetic acid is 4.76. Hint 1. How to approach the problem To solve this problem, first calculate the number of moles of conjugate acid and conjugate base in the initial buffer. Next, determine the effect of the addition of the acid on the total number of moles of conjugate acid and conjugate base. Finally, use the Henderson-Hasselbalch equation to determine the new pH, and compare this value to the initial pH to determine the pH change. Hint 2. Determine the initial number of moles of conjugate acid How many moles of conjugate acid are in the initial buffer? Hint 1. Calculate the total number of moles of acid + base How many moles of conjugate acid plus conjugate base are in the buffer? Express your answer numerically. moles of acid + base =1.65×10 −2 Hint 2. Calculate the ratio of base to acid What is the ratio of conjugate base to conjugate acid in the original buffer? Express your answer numerically, as a decimal number. =1.74 Hint 3. How to find the initial quantity of acid You now have two equations with two unknowns, which can be solved
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Base/Acid Ratios in Buffers Just as is the negative logarithm of , is the negative logarithm of , The Henderson-Hasselbalch equation is used to calculate the of buffer solutions: Notice that the of a buffer has a value close to the of the acid, differing only by the logarithm of the concentration ratio .
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This note was uploaded on 08/12/2008 for the course UG 07120_02 taught by Professor Stueben during the Summer '08 term at Logan University.

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General Chemistry II - Assignment 4 - [ Print View ]...

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