Summary

Summary unit 13: Learning aim A acid-base equilibria in order to understand buffer action and to optimise acid-base titration procedures

0 purchase

Module
Unit 13 - Applications of Inorganic Chemistry

Institution
PEARSON (PEARSON)

My assignment covers all key aspects of acid-base equilibria, including strong and weak acids, alkalis, and buffer solutions. I've explained fundamental concepts such as Brønsted-Lowry theory, acid dissociation constants, and pH calculations. Practical applications in industries like wastewater tr...

[Show more]

Preview 2 out of 13 pages

View example

Uploaded on March 16, 2024
Number of pages 13
Written in 2023/2024
Type Summary

Task 2: Determining the acid dissociation constant (Ka) for a weak acid (A.P2)

Aim
The purpose of this experiment is to accurately demonstrate a reading of pH of a
half-neutralised solution of ethanoic acid and from this determine a value for Ka.

Introduction
You will first accurately determine the titre of sodium hydroxide solution required to
neutralise 25.0 cm3 of 1.0M ethanoic acid. You will then neutralise exactly half of a new
sample of 25.0 cm3 of ethanoic acid with sodium hydroxide and accurately measure the pH
of the resulting solution in order to determine Ka.

Chemicals and Equipment
➔ ethanoic acid (approx. 0.1M)
➔ sodium hydroxide solution (0.1M)
➔ safety spectacles
➔ 1 burette, 50 cm3, and stand
➔ 1 funnel, small
➔ pipette, 25.0 cm3, and pipette filler
➔ 1 beaker, 250 cm3
➔ 1 conical flask, 250 cm3
➔ distilled water
➔ pH metre (accurately calibrated)

CAUTION:

Sodium hydroxide solution is corrosive. You must wear safety spectacles throughout the experiment.

Method
1. Clamp the 50 cm3 burette to the stand. Using a funnel, rinse and fill the burette with 0.1M sodium
hydroxide solution.
2. Using a pipette filler, rinse the pipette with some of the ethanoic acid and transfer precisely 25.0
cm3 of the solution into a 250 cm3 conical flask.
3. Add 2-3 drops of phenolphthalein indicator solution to the conical flask and swirl to mix.
4. Run sodium hydroxide solution from the burette, with swirling until the solution just turns pink
and is permanent.
5. Repeat the titration until you have obtained concordant titres. Calculate the mean titre (V cm3).
6. Pipette precisely 25.0 cm3 of the ethanoic acid solution this time into a 250 cm3 beaker. Do not
add phenolphthalein indicator.
7. Run exactly V/2 cm3 of sodium hydroxide solution from the burette into the beaker of ethanoic
acid. This is the volume required to neutralise exactly half of the ethanoic acid.
8. Swirl gently to ensure that the solutions are well mixed.
9. Place an accurately calibrated pH metre into the beaker so that the tip of the probe is completely
immersed in the solution you have made.
10. When the reading is stable, read and record the final pH of the solution in the beaker.
11. Repeat the half-neutralisation steps again in order to obtain a reliable average pH reading.

, Results of the experiment

Trial run 1st run 2nd run
3
Initial reading / cm 0.00 0. 00 0.00
Final Reading / cm3 25.1 24.4 24.4
Titre (volume used) / cm3 25.1 24.4 24.4
Mean titre (V) / cm3 24.4
V/2 / cm3 12.3

pH of half-neutralised ethanoic acid solution (1st run) 4.72
pH of half-neutralised ethanoic acid solution (2nd run) 4.72
pH of half-neutralised ethanoic acid solution (3rd run, if required) 4.73
Average pH value 4.72

Calculation

The acid dissociation constant for a weak acid (Ka) is given by the equation:
+ −
𝐾𝑎 = [ [𝐻𝐴]
𝐻 ][𝐴 ]

Therefore, when a weak acid is half-neutralised, the concentration of the weak acid [HA] is equal to the
concentration of its conjugate base [A-] (ie [HA] = [A-]), so the equation for Ka can be expressed as

Ka = [H+] or pKa = pH

The equation can be rearranged such as : Ka = 10-pH

Ka = 10 -4.71

ka = 1.95 x 10-5

∴ The value of Ka is 1.95 x 10 -5

Comparing the ka for ethanoic acid with the published ka value for ethanoic acid:
Therefore, my ethanoic acid value is 1.95 x 10-5 whereas the published value of the ethanoic acid is
slightly different which is 1.74 x 10-5. This suggest that there are some reason why the value is slightly
different from the published value. This can have happened because too much solution was added,
slightly crossing the meniscus line and causing inaccurate results. This also implies that parallel errors
may also arise; these errors happen when the buret's scale is not seen from a perpendicular angle. For
example, misreading the volume of burette can affect the titration result. From above the volume of
solution appears to be lesser, while from below the volume of solution appears to be higher. Moreover,
looking in the wrong spot is also another form of measurement inaccuracy. This is because a concave
curve is formed by a solution which is also known as the meniscus line and the volume is measured at
the bottom of the curve. The volume measurement will be inaccurate if the reading is taken from the
upper regions of the curve. Therefore, to improve the error is by reading the volume of solution at eye
level and at the right angle which makes the experiment more liable and accurate. Additionally, failing
to calibrate the apparatus correctly and failing to use a white tile to examine the colour changes can
lead to contamination and may affect the final result. This is because it is possible to make mistakes
when calibrating the equipment. When starting the practical it is necessary to rinse the burette and

The benefits of buying summaries with Stuvia:

Guaranteed quality through customer reviews

Stuvia customers have reviewed more than 700,000 summaries. This how you know that you are buying the best documents.

Quick and easy check-out

You can quickly pay through credit card for the summaries. There is no membership needed.

Focus on what matters

Your fellow students write the study notes themselves, which is why the documents are always reliable and up-to-date. This ensures you quickly get to the core!

Frequently asked questions

What do I get when I buy this document?

You get a PDF, available immediately after your purchase. The purchased document is accessible anytime, anywhere and indefinitely through your profile.

Satisfaction guarantee: how does it work?

Our satisfaction guarantee ensures that you always find a study document that suits you well. You fill out a form, and our customer service team takes care of the rest.

Who am I buying these notes from?

Stuvia is a marketplace, so you are not buying this document from us, but from seller zonanoori. Stuvia facilitates payment to the seller.

Will I be stuck with a subscription?

No, you only buy these notes for £45.49. You're not tied to anything after your purchase.

Can Stuvia be trusted?

4.6 stars on Google & Trustpilot (+1000 reviews)

66184 documents were sold in the last 30 days

Founded in 2010, the go-to place to buy revision notes and other study material for 15 years now

Start selling

Seller

Summary ·