There's a question on AAMC 3r question #47.. that gives the explanation:
In a titration of RCOOH, the concentrations of RCOOH and RCOO- are equal at the mid-point of the titration. This is often called the half-equivalence point. From the expression for the equilibrium constant of a weak acid HA, when [HA] = [A-], then [H3O+] = Ka and pH = pKa. Table 1 shows the pKa value for a monoprotic acid to be 4.694.88. Answer choice A (4.8) lies in this range, the other choices do not. Alternatively, Figure 1 shows the pH at the half equivalence point of a weak acid to be about 4.8. Thus, answer choice A is the best answer.
Maybe I'm just confused, but I thought the concentrations of the acid and its conjugate base were equal at the actual equivalence point...not the half? Also, I'm also a bit cloudy on the "end point" vs "equivalence point". I know its the point where the indicator should change color...so theoretically, shouldn't they be the same?
thanks in advance!
In a titration of RCOOH, the concentrations of RCOOH and RCOO- are equal at the mid-point of the titration. This is often called the half-equivalence point. From the expression for the equilibrium constant of a weak acid HA, when [HA] = [A-], then [H3O+] = Ka and pH = pKa. Table 1 shows the pKa value for a monoprotic acid to be 4.694.88. Answer choice A (4.8) lies in this range, the other choices do not. Alternatively, Figure 1 shows the pH at the half equivalence point of a weak acid to be about 4.8. Thus, answer choice A is the best answer.
Maybe I'm just confused, but I thought the concentrations of the acid and its conjugate base were equal at the actual equivalence point...not the half? Also, I'm also a bit cloudy on the "end point" vs "equivalence point". I know its the point where the indicator should change color...so theoretically, shouldn't they be the same?
thanks in advance!