Question

$p{{K}_{a}}$ value of acetic acid is 4.76 at room temperature. How do you obtain buffers of 4.4 and 5.4 pH values from acetic acid and sodium acetate?

Answer 1

.The pKa value is used to indicate the strength of an acid. pKa is the negative log of the acid dissociation constant or we can say Ka value. A lower pKa value indicates a stronger acid.

Complete answer:
- pH represents the basicity or acidity of a solution on a logarithmic scale on which value 7 is neutral, higher values than 7 are basic and lower values than 7 are acidic.
The pH is equal to , $-{{\log }_{10}}c$ where, c is the concentration of hydrogen ion in moles per litre.
-We are provided with the value of acetic acid at room temperature= 4.76,
- Firstly let’s see the case to obtain the value of 4.4, we will use the formula,
$pH=p{{K}_{a}}+\log \dfrac{\left[ sodium\text{ }acetate \right]}{\left[ acetic\text{ }acid \right]}$
- Now, we will put all the given values in the equation,

& 4.4=4.76+\log \dfrac{sodium\text{ }acetate}{acetic\text{ }acid} \\\ & \log \dfrac{sodium\text{ }acetate}{acetic\text{ }acid}=-0.36 \\\ \end{aligned}$$ So, we get the ratio of sodium acetate to that of acetic acid as: $$\dfrac{sodium\text{ }acetate}{acetic\text{ }acid}=0.4365$$ \- Secondly we will discuss about to obtain pH of 5.4 value: -We will solve it also in similar manner as we have solved in above question, $$pH=p{{K}_{a}}+\log \dfrac{\left[ sodium\text{ }acetate \right]}{\left[ acetic\text{ }acid \right]}$$ $$\begin{aligned} & 5.4=4.76+\log \dfrac{\left[ sodium\text{ }acetate \right]}{\left[ acetic\text{ }acid \right]} \\\ & 5.4=4.76+\log \dfrac{\left[ sodium\text{ }acetate \right]}{\left[ acetic\text{ }acid \right]} \\\ \end{aligned}$$ \- So the ratio of sodium acetate and acetic acid at a pH of 5.4 will be: $$\dfrac{sodium\text{ }acetate}{acetic\text{ }acid}=4.364$$ \- Hence, by this method we can obtain the buffers of 4.4 and 5.4 pH values from acetic acid and sodium acetate **Note:** \- Strong acids or bases will have low values $p{{K}_{a}}$, whereas weak acids and bases will have $p{{K}_{a}}$higher values. Acids are having pH values less than 7 and bases are having pH values more than 7.