# Half-Equivalence Point

#### sotired

##### sotired
10+ Year Member
There's a question on AAMC 3r question #47.. that gives the explanation:

In a titration of RCOOH, the concentrations of RCOOH and RCOO- 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 4.694.88. 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?

#### WilliamsF1

10+ Year Member
The equivalence point for a monoprotic (one H) acid is where it's been 100% deprotonated. i.e. an acid starts as HA and then a strong base (say OH-) is added, when 50% of it gets deprotonated, [HA]=[A-], that is the half equivalence point. If you keep adding OH-, you will deprotonate the remaining acid. Once all the HA has been converted to A-, that is the equivalence point. End point is for indicators and the pH range in which they function.

OP
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#### sotired

##### sotired
10+ Year Member
WilliamsF1 said:
The equivalence point for a monoprotic (one H) acid is where it's been 100% deprotonated. i.e. an acid starts as HA and then a strong base (say OH-) is added, when 50% of it gets deprotonated, [HA]=[A-], that is the half equivalence point. If you keep adding OH-, you will deprotonate the remaining acid. Once all the HA has been converted to A-, that is the equivalence point. End point is for indicators and the pH range in which they function.
ahhhh thanks....i get it now

#### WilliamsF1

10+ Year Member
If you look at the graph (I just brough out 3R), the half-equivalence points will be on the most horizontal line drawn through the graph. The equivalence point is the most vertical line drawn on the graph. So from the graph, the half equivalence is the first flat portion of the graph starting from the lower left. It will be when 15mL has been added at around a pH of 5 so 4.8 is the best answer.

#### EBI831

##### legend in the making
10+ Year Member
7+ Year Member
everythings WilliamsF1 said but add that equivalence pt and end point aren't the same always especially if you were doing a polyprotic acid titration with a base then you'd have several equivalence points but just one end point. as far as end point goes i just know you use an indicator and its when you are done and i might be wrong but i think it's when you've added as many equivalents of titrant(take base for instance) to the acid(if acid being titrated) as present in the original acid and the end point may or may not be at a neutral pH(usually only when strong base and strong acid or weak acid and weak base). don't shoot me if i'm wrong just pat me on the back for trying.