Sprinkles

Thank you for the reply! It will have a charge of 0, but how would this affect the total number of moles of H+ needed? For example, we would still need to do two H-H equations (1st H-H: use pKa of COOH, 2.1 and pH, 3.2 / 2nd H-H: use pKa of NH3+, 9.6 and pH 11). Find "a" by using the 1st H-H, and find "b" using the 2nd H-H. The total number of moles of Histidine= 0.4 L x 0.2 M= 0.08 moles of Histidine [*Assuming that the addition of acid doesn't interfere w/volume of solution.*] To find the total number of moles of H+ = ("a")(0.08) + ("b")(0.08) (+ or -) _______ I'm getting stuck with the last blank. Would I subtract or add 0.08 now? Edit: To find the moles of OH- needed to change pH from 3.2 to 11, you would add another 0.08 moles of Histdine (in that last blank). So, to find the moles of H+ needed to change pH from 11 to 3.2, does this mean that you need to subtract 0.08 moles of Histidine? I am getting confused, because I get the same answer for both the number of moles of OH- needed to go up the curve, and for the number of moles of H+ needed to go down the curve. My reasoning is because it's all along the same curve, and it's an equilibrium. Therefore, shouldn't the necessary moles of H+ and OH- be the same?

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. -------- 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!