CHM 114 Study Guide - Final Guide: Magnetic Stirrer, Ph Meter, Equivalence Point

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Determination of ka of an unknown weak acid. The purpose of this experiment is to titrate an unknown weak acid with the known strong base. Small amounts of naoh will be added to the unknown acid to find the ka of the unknown solution. In this experiment we can determine this in two ways. The first is to find the ka from the ph of the original diluted solution and the second is by finding the ka from the ph at the half- neutralization point. In order to find the ka we must understand what it stands for. When a weak acid is dissociated in water, a salt and [h3o+] is formed. Ka is the dissociation constant for an acid and determines the strength of the weak acid. We can determine the ka by measure of the hydronium ion. [h3o+], the salt [a-], and the original concentration of the weak acid.

Related Questions

I performed an experiment to find the equivalent mass anddissociation constant of an unknown weak acid by titrimetry.

My data is the following:

A.) Determining a satisfactory sample size for the unknown weakacid:

Mass of unknown acid, 0.12 grams

Final buret reading: 5.6 mL

Initial buret reading: 0.0mL

Volume of NaOH solution required: 5.6mL

Mass of unknown acid requiring 25mL of NaOH solution fortitration: 0.54grams

Volume of NaOH solution required: 300mL

Molarity of NaOH solution: 0.25M

B.) Titrating a sample of an unknown weak acid

Mass of weighing paper plus unknown acid: 0.91grams

Mass of weighing paper: 0.37 grams

Mass of unknown acid: 0.54grams

C.) Buret readings of first and second determinations:

First determination:

13.5mL = 3.82 pH

26.5mL = 7.60 pH

Second determination:

13.5mL = 3.74 pH

27.0mL = 7.60 pH

pKa of acid: 3.82 / 3.74

Ka: 1.85x10^-4 / 1.82x10^-4

Find the following for both determinations 1 and 2:

Volume of NaOH solution required to reach equivalence point, mL:?

Number of equivalents of NaOH required to reach equivalencepoint: ?

Number of equivalents of unknown acid titrated, equiv: ?

Equivalent mass of unknown acid, g/equiv: ?

Refer to this equation: Ka = [H3O+]halfway [A-]halfway / [HA]halfway = [H30+]halfway

and consider carefully whether you could simply have used the pH value of the solution at the very beginning of the titration, before any base had been added, to determine either or both the molar mass and the Ka value. Which of the following is the best conclusion?

A) The intitial solution pH depends only on the concentration of the weak acid, so measuring the initial pH is sufficient to determine the concentration. The Ka can then be calculated.

B) The initial solution pH depends only on the Ka of the weak acid, so measuring the initial pH is sufficient to determine the Ka. The concentration can then be calculated.

C) The initial solution pH depends on both the Ka and the concentration of the weak acid, so measuring the inital pH is insufficient to determine both these unknowns.

D) The initial solution pH depends on both the Ka and the concentration of the weak acid, so measuring the initial pH very accurately may be sufficient to determine both unknowns.

The concentration of NaOH is 0.0829 M

CHM 114 Study Guide - Final Guide: Magnetic Stirrer, Ph Meter, Equivalence Point

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