# 11.13 - for preparing a buffer pH = 7.0 2 What is the pH of a buffer formed from 50 mL of 15.0 M NH 3 and 53.5 g of NH 4 Cl in enough water to make

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Chem 102B – Discussion Worksheet #21 Buffers A buffer solution is one which maintains an approximately constant pH when small amounts of either a strong acid or base are added. Solutions containing either weak acids and their conjugate bases, or weak bases and their conjugate acids, can be buffering solutions. The Henderson- Hasselbalch equation is typically utilized to calculate the pH of buffer solutions. Properties of buffer solutions: 1) The number of moles of the conjugate pairs must be large compared to the moles of added strong acids or bases. 2) The ratio of the conjugate pair ([weak acid]/[conjugate base]) should lie between 0.1 and 10, with optimal buffering at a 1:1 ratio. 3) The pH of a 1:1 ratio buffer is equal to the p K a of the weak acid or p K b of the weak base. The effective range is +/- 1 from the p K a or p K b . ACTION ITEMS CH 3 COOH/CH 3 COO - K a = 1.8 x 10 -5 NH 3 /NH 4 + K b = 1.8 x 10 -5 HCO 3 - /CO 3 2- K a = 5.6 x 10 -11 H 2 PO 4 - /HPO 4 2- K a = 6.2 x 10 -8 1. Which of the conjugate pairs, shown above would be appropriate

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Unformatted text preview: for preparing a buffer pH = 7.0? 2. What is the pH of a buffer formed from 50 mL of 15.0 M NH 3 and 53.5 g of NH 4 Cl in enough water to make a solution with a total volume of 500 mL? 3. What is the pH of the previous solution after addition of 100 mL of 0.2 M NaOH? Buffers can also be made by partially neutralizing a weak acid with a strong base, or a weak base with a strong acid. 4. Which of the following mixtures result in a buffering solution when equal volumes of the two solutions are mixed? a. 0.10 M HCl and 0.10 M NH 4 Cl b. 0.10 M HCl and 0.10 M NH 3 c. 0.20 M HCl and 0.10 M NH 3 d. 0.10 M HCl and 0.20 M NH 3 5. What is the pH of a solution made by adding 0.40 g of NaOH to 100 mL of 0.4 M solution of CH 3 COOH? Is it a buffering solution? 6. What volume of 2.0 M NaOH must be added to 100.0 mL of 2.5 M CHOOH ( K a = 1.8 x 10-4 ) to prepare a buffer with a pH of 4.0?...
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## This note was uploaded on 02/14/2011 for the course CHEMISTRY 102 taught by Professor Whitt during the Spring '11 term at University of Illinois, Urbana Champaign.

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11.13 - for preparing a buffer pH = 7.0 2 What is the pH of a buffer formed from 50 mL of 15.0 M NH 3 and 53.5 g of NH 4 Cl in enough water to make

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