Gen Chem II Lab 17: Titration Curves of Polyprotic Acids

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Last updated 1:07 AM on 4/8/26
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25 Terms

1
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polyprotic acid

an acid that can lose more than one per molecule in an acid base reaction

<p>an acid that can lose more than one per molecule in an acid base reaction</p>
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why is pKa2>pKa1?

the second proton disassociation involves removing a proton from a already negatively charged species. Thus, there is greater resistance faced in removing the second proton.

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why is Ka1>Ka2 ?

The conjugate base and acid resulting from the first proton disassociation have more H+ ions (more acidic) than the conjugate base and conjugate acid in the proton disassociation

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why don’t we see the third equivalence point ?

the acid strength of HPO42-(conjugate acid of the final ion PO43-) and as we add water the concentration of the acid is diluted

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<p>what is the first line pointing to</p>

what is the first line pointing to

buffer region 1

<p>buffer region 1</p>
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<p>at buffer region 1, what is in the solution</p>

at buffer region 1, what is in the solution

a mix of H3PO4 and H2PO4-

<p>a mix of H<sub>3</sub>PO<sub>4 </sub>and H<sub>2</sub>PO<sub>4</sub><sup>-</sup></p>
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<p>what is the first dot pointing to</p>

what is the first dot pointing to

the 1st equivalence point

<p>the 1st equivalence point</p>
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molecular equation for reaction at 1st equivalence point

H3PO4(aq)+NaOH(aq)→NaH2PO4(aq)+H2O(l)

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<p>what is the second line pointing to</p>

what is the second line pointing to

buffer region 2

<p>buffer region 2</p>
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<p>what is in the solution in buffer region 2?</p>

what is in the solution in buffer region 2?

a mix of H2PO4- and HPO2-

<p>a mix of H<sub>2</sub>PO<sub>4</sub><sup>-</sup> and HPO<sup>2-</sup></p>
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<p>what is the second dot pointing to ?</p>

what is the second dot pointing to ?

the second equivalence point

<p>the second equivalence point</p>
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molecular equation for reaction at second equivalence point

NaH2PO4(aq)+NaOH(aq)→Na2HPO(aq)+H2O(l)

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<p>what is the third line pointing to ?</p>

what is the third line pointing to ?

buffer region 3

<p>buffer region 3</p>
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what is in the solution in buffer region 3?

a mix of HPO42-and PO43-

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<p>what is the third dot pointing to ?</p>

what is the third dot pointing to ?

equivalence point 3

<p>equivalence point 3</p>
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molecular equation at equivalence point 3

Na2HPO4(aq)+NaOH(aq)→Na3HPO4(aq)+H2O(l)

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<p>what is in the titration in the third dot ?</p>

what is in the titration in the third dot ?

100% PO43-

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<p>what is in the titration at the second dot</p>

what is in the titration at the second dot

100% HPO42-

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<p>what is in the titration at the first dot</p>

what is in the titration at the first dot

100% H2PO4-

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pKa→Ka

Ka=10-pKa

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how to find equivalence point from second derivative graph

take the point where the parabola crosses 0, that is the volume at ½ the equivalence point, double it to find vol at equivalence point

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determining concentration of acid from equivalence pt

vol at equivalence point x [NaOH]= mol NaOH =mol acid, mol acid /vol in L= [acid]

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Ka

the acid disassociation constant

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the higher the Ka the greater

disassociation into products

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Ka expression

= [A-][H3O+]/[HA]