1. The problem statement, all variables and given/known data There is 400 mL of 0.2 M histidine buffer- pH 11. Find the moles of HCl that are needed to change the pH to 3.2. 2. Relevant equations pH= pKa+log/[a] 3. The attempt at a solution First, this is how you would go from acid to base (acid-->base): 1. There is 400 mL of 0.2 M histidine buffer- pH 3.2 Find the moles of OH that are needed to change the pH to 11. H+2 <--> H+1 <-->H0 <-->H-1 1) Find a and b using H-H, and pKa values (2.1-pKa of COOH and 3.2 in one equation, and 9.6-pKa of NH3+ and 11 in the second equation). ADD PLUS ONE. (H+1<---->H0) a+b+1= total1) [This total is the total # of equivalents.] 2) Multiply .4 L by 0.2 M.=total2) 3) Multiple total1) and total2) together to get the final answer. Now my question: What if we had to go backwards- from base to acid? There is 400 mL of 0.2 M histidine buffer- pH 11. Find the moles of HCl that are needed to change the pH to 3.2. Would I subtract -1 now? (Please see 1).) I am just confused on how to go backwards (base to acid). How should the number change? When going from a-->b you need moles of OH- to increase the pH, but from b-->a you would need moles of HCl to decrease the pH, so the number of moles should be different, I assume. However, when calculating it, I get the same number of moles for both H+ and OH-. Thank you! Thank you!