The scenes below depict the relative concentrations of H3 PO4, H2 PO4^-, and HPO4^2- during a ti

The scenes below depict the relative concentrations of H3...

The scenes below depict the relative concentrations of $\mathrm{H}_{3} \mathrm{PO}_{4}, \mathrm{H}_{2} \mathrm{PO}_{4}^{-},$ and $\mathrm{HPO}_{4}^{2-}$ during a titration with aqueous $\mathrm{NaOH}$ , but they are out of order. (Phosphate groups are purple, hydrogens are blue, and $\mathrm{Na}^{+}$ ions and water molecules are not shown.) (a) List the scenes in the correct order. (b) What is the pH in the correctly ordered second scene (see Appendix $C ) ?(c)$ If it requires 10.00 $\mathrm{mL}$ of the NaOH solution to reach this scene, how much more is needed to reach the last scene?

Key Concepts

Equivalence Point and Stoichiometry

The equivalence point in a titration is reached when the amount of titrant added is chemically equivalent to the substance being titrated. In titrations involving polyprotic acids, there are multiple equivalence points corresponding to each deprotonation step. A clear grasp of stoichiometry is required to relate the volume of titrant added to the changes in species concentrations and to calculate additional titrant volumes needed to complete the reaction.

Henderson-Hasselbalch Equation

This equation relates the pH of a buffer system to the pKa of the weak acid and the ratio of the concentrations of the conjugate base to the acid. It is particularly useful in titration problems to calculate the pH at certain stages, such as at the center of a buffering region, where the acid and its conjugate base are present in nearly equal amounts.

Buffer Regions

Buffer regions occur during titrations where a weak acid and its conjugate base coexist in appreciable amounts, resisting drastic changes in pH. In polyprotic acid titrations, the regions around the pKa values act as buffers, and the pH in these regions can be predicted effectively. Recognizing these zones is essential for calculating pH and interpreting titration curves.

Polyprotic Acid Titration

This concept involves acids that can donate more than one proton (H+), undergoing multiple deprotonation steps during titration. Each step has its own equilibrium constant (pKa value) and distinct titration curve region. Understanding these sequential reactions is key to determining the order of species present and predicting the titration behavior of substances such as phosphoric acid.

Transcript

00:01 In this image, we have four stages of phosphoric acid being titrated with noh.

00:07 What that means is we have phosphoric acid in this beaker down here.

00:13 So phosphoric acid or h3po4 is the anolyte, and the titrant is n -a -o -h, which is in the burette up here.

00:23 Our goal is to arrange a, b, c, and d into chronological order.

00:27 Our first step at the very beginning of the titration before any noh is added, we have a beaker full of just h3po4.

00:39 The red spots are phosphate groups, and the smaller blue ones are hydrogens.

00:48 So these images right here with one large red spot and three smaller blue ones is the original phosphoric acid that we begin.

01:00 With.

01:02 So we know that our initial beaker will look like c with all h3 -p -o -4 molecules.

01:13 Once we start adding naoh, the hydroxide ions from the n -a -o -h will react with h3 -p -o -4 to create water and h2p -o -4 -minus.

01:41 So when we start adding an a .o .h.

01:45 We'll have some h3p .o4 and some h2p .o4 minus in our beaker.

01:51 And that is beaker b.

01:54 So that's our second step.

01:57 In this, if we keep adding an aoh, it will react with all of the h3po4, and we'll eventually be left with only h2po minus.

02:09 Then the excess hydroxide will react with that h2po4 minus to form more water and hpo4 -2 -minus.

02:32 And in this stage we'll have some h2 -04 and some hpo4 -2 -minus, which is stage d...

Please give Ace some feedback