Research Question By measuring the pH value of the acetic acid using a pH meter at standard lab conditions, will increasing the concentration of the acid affect its experimental determined Ka and therefore its calculated pKa?

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High Level Chemistry

Design


Investigate the effect of concentration on the experimentally determined pKa of a weak acid

        


Background Information

An acid is a proton donor which means it is able to donate a proton to another substance. The substance that accepts the proton from the acid is known as a base. An acid can be categorised into being a weak or a strong acid. A weak acid is an acid which does not completely dissociates. In other words, a weak acid does not completely ionise when it is dissolved in aqueous solutions.

The pKa, also known as Dissociation Constant, is a value that determines the strength of an acid or base. The pKa value of an acid or base is largely related to the pH of the substance. [1] pH is a value given between 1 and 14 to identify the concentration of hydrogen ions in an aqueous solution.

pH is found by calculating the negative logarithm of the hydrogen activity in the solution. [3]  

pH = -log aH

where aH denotes activity of hydrogen ions

The dissociation can be written as

Ka = [A-][H+]

        [HA]

The unit of Ka is molar per decimetres cubed (mol/dm3)

Like the relationship between pH and the hydrogen ion activity, pKa is also found bye taken the negative logarithm of Ka. [1]

pKa = -log Ka

The pKa values of weaker acids will be larger than the values for stronger acids. Therefore the higher the pKa number is the weaker the acid. The concentration of an acid affects the Ka value and therefore the pKa value for a high molarity of any solution will attain a lower pH. With references to Ka, as the molarity of the solution increases the value of Ka decreases and the smaller Ka is the bigger pKa is. [2]

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Calculating Ka and pKa  [2]

Dissociation equation for the acid:        HA   ↔   H+   +   A¯

Equilibrium expression:                         Ka   =   (  [H+]  [A¯]  )   /   [HA]

pH           =   -log [H+]   →   [H+]   =   10¯pH 

[H+]           =   x  Molar

1:1 molar ratio between [H+] and [A¯]   →   [A¯]   =   [H+]

Ka           =   [ (A¯) (H+) ]   /   Molarity of solution

pKa           =   – log10 (Ka)

Ethanoic acid is a typical weak acid ...

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