Volumes of the equivalence points; then you uaon rapldly so that you can do thè
ID: 636509 • Letter: V
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Volumes of the equivalence points; then you uaon rapldly so that you can do thè next two titrations with large-volume lence points and small-volume increments near the equivalence points.) Before beginning this experiment in the labora increments away from the equiva- tory, you should be able to an-| PR Qt swer the following questions: 1. What is a polyprotic acid? 2. If 10.7 mL of 0.110 M NaOH is required to reach the first equivalence point of a solution of citric acid (H&CsHO;), how many mL of NaOH are required to completely neutralize this solution? How many mmol of NaOH will react completely with 50 mL of 1.8 M 3. 4. How many moles of H,O' are present in 250 mL of a 0.20 M solution 5. 6. If the pH at one half the first and second equivalence points of a dibasic of H2SO4? Why is it necessary to standardize a pH meter? acid is 4.20 and 7.34, respectively, what are the values for pkl pK 2? From pKat and pka2 calculate the Ka1 and Kd2 Derive the relationship between pH and pKa at one half the equiva- lence point for the titration of a weak acid with a strong base. Could K, for a weak base be determined in the same way that Ka for a weak acid is determined in this experiment? 7. 8. 9. What is the relationship of the successive equivalence-point volumes in 10. If the pK,1 of a diprotic acid is 2.90, what is the pH of a 0.10 M solution the titration of a polyprotic acid? of this acid that has been one quarter neutralized?Explanation / Answer
1. Polyprotic acid=
Polyprotic acid is a type of acid which has more than one proton. Polyprotic acid is capable of losing more than one proton per molecule.
For example,
Monoprotic = HA (HCL)
Diprotic = H2A (H2CO3)
Triprotic = H3A (H3PO4)
5. The standardization of pH meter can be done with the help of standardize buffer solutions of pH 9.2, pH 4 and pH 7. These buffer solutions shows gurenteed pH values respectively. It is very necessary to standardize the pH meter before use to get accuracy in results. Without standardize pH meter the results may be wrong or inacurrate. The standardization of pH meter is important to get continues accuracy in your results. The pH meter is based on the potential differnce inside KCl soluition and external solution. so the potential may be change due to the strength of kcl solution and due to external sources which may contact with the electrode. In this case, the pH meter will not show the exact or true value. so we have to standardize the pH meter to see it showing correct values or not.
7. Relationship between pH and pKa=
pH is the concentration of H+ ions in the solution and pKa is the acid dissociation constants which tells you that what pH need to donate the protons.
Lower pH = Higher H+ ions concentration
Lower pKa = Strong acid, Higer capacity to donate protons.
pH depeds upon the concentration of solution means weak acid has low pH. While pKa does not depend upon concentration. pKa value is constant for each type of molecule.
The relationship between pH and pKa of weak acid is given by Henderson-hasalblach equation which states that, "The pH is the sum of the pKa value and log of the concentration of conjugate base divided by the concentration of weak acid."
pH = pKa + log [conjugate base ] / [weak acid]
pH = pKa + log [A-] [HA]
At half equivalence point,
pH = pKa.
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