A 40.0 mL sample of a mixture of HCl and H3PO4 was titrated with 0.100 M NaOH. The first equivalence point was reached after 88.0 mL of base, and the second equiva-lence point was reached after 126.4 mL of base. (f) What indicators would you select to signal the equiva-lence points?

Video transcript

Hello. Everyone in this view of being told that we have a sample mixture of hydrochloric acid and arsenic acid with a volume of 50 ml. As being tight traded with sodium hydroxide. So being asked basically to find out which indicators would be most suitable to signal the equivalence point that the 1st and 2nd equivalents points were reached after 95 mm and 140 ml of the base were consumed respectfully. So basically we're saying are being told that 100% of the hydrochloric acid is being neutralized at the first equivalence point. Now for our arsenic acid, which is H three A. S. 04, we're gonna go ahead and list out some of our peak values. So the P K A one is equal to 2.26. R p K A two is equal to 6.76 and I peek A three is equal to 11.29. So the selection for the most suitable indicator is based on the p K I N. Or sub I. N. Which is just the p. K. Of the indicator. So the ph at the equivalence point should be close to the p K. Of the indicator. So the ph of the first equipment's point is equal to the average of r p K one and K two of arsenic acid. So let's go ahead and calculate that. So the p H at our first equivalence point. Put E. Q here and point so finding the average we set is P. K. A. One and P. K. A two divided by two. So following this formula. then we'll plug in P K one that's 2. and P K two is 6.76, all divided by two. Once you put that into the calculator, we see that the ph at the first equivalence point is equal to 4.51. So the most suitable indicator to signal that the first opponents point was reached is going to be. So we'll go ahead and work this out that the indicator and D K tour is equal to. So it is our ethyl orange. And the reason for that is because the The PK of the indicators of PKIN is equal to 4.34. So this is very close. So this is going to be my answer for our first equivalence point. Now the ph for the second equivalence point is equal to the average of R P k two and K three. So P H At 2nd equivalence point Again, that's equal to average over PKA two and PKA three formula vintages PK two plus PK three over to the same thing as above. Just with different P K. So for P K two, that's 6.76 and P K three is 11.29. Go ahead and divide this by two. So once I put that I calculated, I see that the ph at the second equivalence point is equal to 9.25. So then the most suitable indicator to signal that the second equivalence point was reached is going to be so my indicator indicator is equal to find more blue. And the reason for this is because the P. K. Of our indicators of p k sub I N is equal to 9.20, and this value is the closest to our ph So this right here is going to be my answer for our second equivalence point. So again, the indicator to signal for our first equivalence point is at the orange and the indicator for a second equivalence point is going to be diamond blue. Thank you all so much for watching.