Below are partial solutions to the problems done in class.
- For the last question done in class,1998 #1 b) i), there was an error. We (I) forgot to square the value for the hydroxide concentration, thus the value for the zinc ion concentration was wrong. The correction is in the solution below. I caught the mistake with a student when we did the confirmation of "Q" when we plugged in the concentration values, so the lesson is always confirm your answers if you have the time.
- For part (d), 1996, there was some confusion. In the solution below, I showed how you could use Henderson-Hasselbalch to solve for the concentration of the conjugate base. Since both the pH and pKa are the same, subtracting pKa from pH would leave you with zero. To solve for "x", the inverse of log has to be performed. This would make the zero on one side of the equation "1" and possible to solve for.
- Part (d), 1996 can also be solved by knowing the concept that pH is equal to pKa when you are at the half-way point of a titration. That is when the amount of conjugate base is equal to the original acid. This ratio would be one. The log of 1 is zero, thus pH is equal to pKa. The problem gave the concentration and volume of the acid, so the millimoles of the acid at the equivalence point can be determined. For the problem, 10.0 millimoles of acid would be at the equivalence point. Therefore, half of 10.0 millimoles is 5.0 millimoles, which is the amount of NaOH that would be added to get the ratio of acid to conjugate base just right to have the desired pH.
The solutions for the day 3 problems will be on the Events Calendar tomorrow morning. Remember to finish the two problems on the day 4 packet for tomorrow that have $ signs by them. There are two extra problems, but those are optional to do. I probably will not have the Podcasts done for the two extra problems until Saturday. The last early morning review session is tomorrow at 6:30 am. We will be doing two problems during the session.
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