Which of the following solutions will produce a buffer with a pH near 8.50?

0.100 L 1.00 M H3BO3 (Ka = 5.8E-10) + 0.020 L 2.50 M KOH.

0.100 L of 0.50 M HONH2 (Kb = 1.1E-8) + 0.50 L of 0.050 M HBr.

2.00 L 0.50 M H2NNH2 (Kb = 3.0E-6) + 2.00 L 0.25 M HNO3.

1.00 L of 0.50 M HSO4- (Ka = 1.0E-7) + 1.00 L of 0.25 M HCl.

0.500 L 1.00 M CH3NH2 (Kb = 4.4E-4) + 1.00 L 0.25 M HNO3.

When I calculate each of these I get pH's around 12! Can someone explain how to go about solving this?

## Answers (1)

without having to solve each problem with concentration; you can use your knowledge of Ka to figure out the answer.

As you know pKa + pKb = 14. To get pKa you take the negative log of your Ka and to get pKb you take the negative log of you Kb. After that you use basic algebra and plug your answer into the equation above. Once you find a value near 8.5 then that is your answer.

I will let you solve using the steps I provided to you. I found the answer in about 10 seconds with this method.

Additional information you should know. A good buffer is plus or minus 1 pH of the solution. The more negative the Ka the more acidic the solution is...-9 to -14 is acidic, -6 to -8 is neutral and 1 to -5 is going to be basic. This is handy to know so you can answer questions like these by just looking at them.

Reference:chem tutor