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pH, Buffers, Weak Acid Ka, and Kb Values


Imagine that you are in chemistry lab and need to make 1.00L of a solution with a pH of 2.50. You have in front of you:

- 100mL of 7.00x10-2 M HCl

- 100mL of 5.00x10-2 M NaOH, and

- Plenty of distilled water.

You start to add HCl to a beaker of water when someone asks you a question. When you return to your dilution, you accidentally grab the wrong cylinder and add some NaOH. Once you realize your error, you assess the situation. You have 82.0 mL of HCl and 85.0 mL of NaOH left in their original containers.

Assuming the final solution will be diluted 1.00L, how much more HCl should you add to achieve the desired pH?

Express your answer numerically in milliliters to three significant figures.

_________________________ mL


You need to produce a buffer solution that has a pH of 5.42. You already have a solution that contains 10 mmol (millimoles) of acetic acid. How many millimoles of acetate (the conjugate base of acetic acid) will you need to add to this solution? The pKa of acetic acid is 4.74.

Express your answer numerically in millimoles to three significant figures:

_______________________ mmol acetate


A new potential heart medicine, code-named X-281, is being tested by a pharmaceutical company, Pharmapill. As a research technician at Pharma-pill, you are told that X-281 in a monoprotic weak acid, but because of security concerns, the actual chemical formula must remain top secret. The company is interested in the drug's Ka value because only the dissociated form of the chemical is active in preventing cholesterol buildup in arteries.

To find the pKa of X0281, you prepare a 0.075 M test solution of X-281. The pH of the solution is determined to be 2.70. What is the pKa of X-281?

Express your answer numerically.

pKa = _____________________________________


Given that

- Ka for HCN is 4.9 x 10-10 and
- Kb for NH3 is 1.8 x 10 -5,


- Kb for CN-
- Ka for NH4+

Enter the Kb value for CN- followed by the Ka value for NH4+, separated by a comma, using two significant figures.


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Solution Summary

This solution describes:

1) How to use specific volumes of strong acids and bases to make a solution with a specific pH.

2) How to make a buffer solution.

3) How to find the Ka based on the concentration and pH of a weak acid

4) How to find Kb based on a Ka value.