I keep getting an incorrect answer for problems that use the equation [H+]=Ka[HA]/[A-], however when using the forbidden Henderson-Hasselbalch equation, I get the correct answer. I've checked my algebra many times and have used both moles and M for HA and A-, but I still end up getting the wrong answer. For example, I have tried 4 different approaches to solving #51 in chapter 17, but I keep getting an answer of 1.56g although the textbook's answer is 1.2g.

Help.....It has nothing to do with using the he-ha equation or not. The textbook's answers to several problems, especially those involving HF, are different depending on what value is used for Ka. In this on, try using 3.5 x 10^-4 for Ka.