Docsity
Docsity

Prepare for your exams
Prepare for your exams

Study with the several resources on Docsity


Earn points to download
Earn points to download

Earn points by helping other students or get them with a premium plan


Guidelines and tips
Guidelines and tips

Determination of Ka of Several Weak Acids Lab, Lab Reports of Chemistry

To determine the acid ionization constant, KC, for acetic acid and an unknown monoprotic acid by using indicators and by using a pH meter

Typology: Lab Reports

2020/2021

Uploaded on 05/11/2021

prindhorn
prindhorn 🇺🇸

4.6

(11)

279 documents

1 / 4

Toggle sidebar

This page cannot be seen from the preview

Don't miss anything!

bg1
DeterminationofKaofseveralweakacids
OBJECTIVE
Todeterminetheacidionizationconstant,KC,foraceticacidandanunknownmonoproticacid
byusingindicatorsandbyusingapHmeter.
CONCEPTTOBETESTED
ThevalueofKa,foraweakacidcanbedeterminedfromtheconcentrationsofthespecies
presentatequilibrium.
TEXTREFERENCES
(1)Whitten,K,W.,Davis,R.E.,andPeck,L.GeneralChemistry,5thed.,SaundersCollege
Publishing,Philadelphia,1996,Sections11.1.–11.3,Chapters18and19(2)Safety,pp1–3,(3)
LaboratoryTechniquesSectionKandLandFigureL.1.
INTRODUCTION
Theequation,H2O(l)+HA(aq) H3O+(aq)+A(aq),describestheequilibriumconditions
whenaweakacid,HA,dissolvesinwaterandionizestoformH3O+andKions.Theratioofthe
algebraicproductoftheconcentrationsoftheionstotheconcentrationoftheunionizedacidis
constantandisgiventhesymbol,Kc.

HA
AOH
K
-
3
A
(Eq.1)
Kcappliestoequilibriumconditionsoftheionizationreaction.Iftheacidisrelativelystrong,the
extentofionizationisreflectedinarelativelylargevalueforKc.AlargevalueofKcistheresult
oflargeconcentrationsofH3O+andAionsdividedbythesmallerconcentrationofunionized
HAmolecules,AweakeracidhassmallerconcentrationsofH3O+andAionsandalarger
concentrationofHAmolecules.ItsKcvaluewillbesmaller.ThevaluesofKcfordifferentacids
tellusabouttheirrelativestrengthsasacids.
Therearemanyimportantweakacids.Forexample,vinegarisadilutesolutionofaceticacid,
CH3COOH.Carbonicacid,H2CO3,istheweakacidformedbyCO2andH2Oincarbonatedbever
ages,inblood,andinmanyothersystems.
ThevalueofKc,foranacidcanbecalculatedfromthemeasuredvaluesoftheconcentrationof
theionizedandunionizedspeciespresentinsolutionatequilibrium.
pf3
pf4

Partial preview of the text

Download Determination of Ka of Several Weak Acids Lab and more Lab Reports Chemistry in PDF only on Docsity!

Determination of Ka of several weak acids

OBJECTIVE

To determine the acid ionization constant, KC , for acetic acid and an unknown monoprotic acid

by using indicators and by using a pH meter.

CONCEPT TO BE TESTED

The value of Ka , for a weak acid can be determined from the concentrations of the species

present at equilibrium.

TEXT REFERENCES

(1) Whitten, K, W., Davis, R. E., and Peck, L. General Chemistry, 5th ed., Saunders College

Publishing, Philadelphia, 1996, Sections 11.1.–11.3, Chapters 18 and 19 (2) Safety, pp1–3, (3)

Laboratory Techniques Section K and L and Figure L.1.

INTRODUCTION

The equation, H 2 O(l) + HA(aq) H 3 O

(aq) + A

  • (aq), describes the equilibrium conditions

when a weak acid, HA, dissolves in water and ionizes to form H 3 O

and K ions. The ratio of the

algebraic product of the concentrations of the ions to the concentration of the unionized acid is

constant and is given the symbol, Kc.

   

HA 

H O A

K

3 A

 (Eq. 1)

K (^) c applies to equilibrium conditions of the ionization reaction. If the acid is relatively strong, the

extent of ionization is reflected in a relatively large value for Kc. A large value of Kc is the result

of large concentrations of H 3 O

and A

  • ions divided by the smaller concentration of unionized

HA molecules, A weaker acid has smaller concentrations of H 3 O

and A ions and a larger

concentration of HA molecules. Its K (^) c value will be smaller. The values of Kc for different acids

tell us about their relative strengths as acids.

There are many important weak acids. For example, vinegar is a dilute solution of acetic acid,

CH 3 COOH. Carbonic acid, H 2 CO 3 , is the weak acid formed by CO 2 and H 2 O in carbonated bever‐

ages, in blood, and in many other systems.

The value of Kc, for an acid can be calculated from the measured values of the concentration of

the ionized and un‐ionized species present in solution at equilibrium.

Example 1: The equilibrium concentrations for a given acid were found to be as follows.

HA H 3 O
  • A

Equil. conc. 1.0 x 10

  • M 1.3 x 10 - M 1.3 x 10 - M
  • 1
  • -3 - 3 C 1.0x 10
  1. 3 x 10 1.3x 10
HA
H O A
K  

K (^) a = 1.7 x 10

The [H 3 O

] in an aqueous solution can be expressed as pH, which is defined as

pH = ‐log [H 3 O

] (Eq. 2)

Equilibrium constants can also be expressed in a similar way.

pK (^) a = ‐ [log Ka ] (Eq. 3)

By taking the negative of the common logarithm of Equation 1, a useful relationship between

pK (^) a and pH called the Henderson‐Hasselbach equation can be derived.

HA 

A log a

pH pK

  (Eq. 4)

If a solution of a weak acid is titrated to its endpoint, the anion, A

  • , species is present in the

form of a salt. If this solution and the acid are mixed to give a 50:50 mixture then [HA] = [A

  • ]. A

solution that contains the weak acid, HA, and the anion, A

  • , of this weak acid from its salt is a

buffer system. In a buffer solution in which [A

  • ] = [HA], the measured pH of the solution equals

to the pK (^) a value. These conditions offer a convenient way to determine the pK (^) a of an acid from

pH measurements. K (^) a , is determined from pK (^) a (Eq. 3).

A. The Use of a pH Meter to Measure pH (See webpage for electrode calibration instructions)

A pH meter gives more accurate measurements of pH than do indicators, but it uses more

expensive equipment and must be calibrated, pH meters are generally similar in their operation

Your instructor will provide some instructions for the use of the computer based pH system.

A pH meter consists of a voltage measuring device and two electrodes. A reference electrode,

usually a calomel electrode, provides a constant potential while glass electrode generates a

potential that is proportional to the pH of the solution. When the two electrodes are placed in a

solution, the voltage is displayed on a meter that is calibrated in pH units.

The pH meter is calibrated against a standard solution of known pH before use. Care must be

used in handing the electrodes. The thin membrane that is permeable to H 3 O

ions is very

C. Determination of Ka Values for an Unknown Monoprotic Acid Using a pH Meter

Step 8. Repeat Part B Steps 2 through 7 using one of the unknown weak monoprotic acid

solutions as you did with acetic acid.

D. Data Analysis

Step 9. Plot your data as pH (vertical axis) versus mL of NaOH solution added

Step 10. From your graphs, determine the exact volume of standard NaOH solution

required to reach the equivalence point and then calculate the concentration of

the acetic acid in your original solution. (Remember: MAVA = MBV (^) B)

Step 11. From your graphs, determine the value of Ka for CH 3 COOH and for the unknown

acid. (Remember: the pK (^) a = pH at the half‐way point of the titration.)