Achieve #2 Question 5

[Devyn Chun 3I]

This is the question: The Kb for an amine is 7.448×10−5. What percentage of the amine is protonated if the pH of a solution of the amine is 9.025 ? Assume that all OH− came from the reaction of B with H2O.

I was a little confused about how to go about doing this problem. Do I need to find the pOH from the pH and then find the variables that make the Kb? I was also confused about how to find the original concentration of amine to use as the denominator in the final fraction to find the protonated percentage.

[Anne Mariano 3F]

You are on the right path as you would need to find pOH using pH. After finding pOH, relate that to [OH-].

Then you need to find the equation using the given reactants and products to find Kb. By finding the chemical equation, you will know that every OH- produced is also one BH+ produced so they are equal to each other. As a result, you will now know BH+.

Using the Kb expression, you will then find B. Lastly, the denominator you will use is the sum of BH+ and B as that is the formal concentration.

[Quynhanh Nguyen 1C]

Hi!

Step 1: use the pH to find the pOH
Step 2: find the OH concentration using 10^-pOH
Step 3: set up your ICE box and Kb equation (it helps to use the ICE box to visualize all the moving parts)
Step 4: find the percent protonated using [HB+]/[B initial] * 100.

There's also a good explanation in our textbook in section 6D!

Hope that helps!