Question

The graph shows the titration curve for 25.0mL of an aqueous solution of B titrated with 0.100MHCl. Which of the following best estimates the initial concentration of the solution of the weak base? (Initial pH of 12 and equivalence point at 50mL with pH of 6)

A. 0.20 M
B. 0.10 M
C. 0.067 M
D. 0.048 M

Answer 1

I believe it is A (since the moles are equal when there are 25ml of B and 50ml of HCl, so the molarity of B would have to be double) but I wanted someone to check my answer.

Answer 2

Assuming that is 50 mL of the HCl so the total is 75 mL (50 mL HCl and 25 mL base) I think you are right. And I think the initial pH of 12 and pH = 6 at the equivalence point is superfluous information; however, I tried to take the 0.2 M for (B) and see if it gives initial pH of 12 or pH = 6 for the eq point. It's close but not those values. I calculated Kb and used that to calculate the eq pt pH. I don't get pH of 6. I don't see a graph so I'm thinking perhaps you read those values from the graph so that pH of 12 and 6 at the two points aren't exactly 12 and 6.