Finding the Ka of Ethanoic Acid
Aim:
====
To find out the Ka of ethanoic acid, chloroethanoic acid and
dichloroethanoic acid.
Procedure:
==========
1. The pH meter is calibrated, using a buffer solution of accurately
known pH.
2. 20.0cm3 of 0.10M ethanoic acid was pipetted into a conical flask.
3. 0.10M sodium hydroxide solution was titrated using phenolphthalein
as indicator, until the solution was just turned pink.
4. A further 20.0cm3 of the same ethanoic acid solution was added to
the flask and was mixed thoroughly.
5. The pH of the resulting solution was determined.
Result:
=======
Volume of NaOH used is recorded below:
Trial (cm3)
1st time (cm3)
Initial burette reading
3.55
18.4
Final burette reading
22.8
36
Titre
19.25
17.6
pH
Chloroethanoic acid
3.0
Dichloroethanoic acid
2.2
Ethanoic acid
4.6
pH of chloroethanoic acid and dichloroethanoic acid is given by the
teacher.
Calculation:
============
Ka =
1. It is reasonable to make the following assumptions:
[HX] » total acid concentration
[X-] » total salt concentration
because CH3COOH is a weak acid which is only slightly ionized and thus
contributed only a little bit to total salt concentration. Therefore
[HX] tends to total acid concentration, and [X-] is mostly contributed
by the salt resulting from neutralization.
Using these assumptions, we have:
[H+] » Ka
2. If the concentrations of the acid and salt in the mixture are
equal, then:
[H+] » Ka ´ 1
[H+] » Ka
3. [H+] = Ka
-log Ka = -log [H+]
pKa = pH
4. The aim of neutralization is to neutralize all the acid, and make
the concentration of the salt same as the concentration of the 20.0cm3
acid. Therefore the titre need not be exactly 20.
In the pH homeostasis lab, 6 experiments were conducted. The hypotheses were: If base is added to water then the pH will increase; If acid is added to water then the pH will decrease; If base is added to homogenate, then the pH will increase; If acid is added to homogenate, then the pH will decrease; If acid or base is added to buffer, then the pH will remain the same. After the experiments were conducted, the graphs were somewhat similar to the hypotheses.
We were then to make a base solution of 0.7 M NaOH. In order to standardize
Apparatus: * 1 measuring cylinder * 1 test tube * 1 stop clock * A large gelatine cube containing indicator and NaOH * Hydrochloric acid ranging from 1-3 molars * A scalpel Diagram: Method: * Take the large gelatine cube and cut into 15 equal pieces * Place on piece of the cube into the test tube * Measure out 10mls of HCl in the measuring cylinder * Pour the HCl into the test tube with the gelatine cube and start the clock * Time how long it takes for the pink colour inside the gelatine cube to completely disappear * You will also notice that the cube dissolves slightly * Record your results and repeat this same process 3 times for each molar of acid: § 1 molar § 1.5 molar § 2 molar
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I decided to experiment with pHs within the range pH 2 to pH7, as I
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As, you can see pure solids and liquids are not included in the Ksp equation because they are already accounted for in the equilibrium constant. A Ksp with a large value has a lot of dissolved ions causing it to have
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