The dissociation constant of a weak acid is $1 \times 10^{-4}$ in order to prepare a buffer solution with a $pH = 5$ the [salt] / [Acid] ratio should be :
- 4 : 5
- 10 : 1
- 5 : 4
- 1 : 10
The Correct Option is B
Solution and Explanation
pH = p K_{a}+\log \frac{[ Salt ]}{[ Acid ]} \\
5=-\log K_{a}+\log \frac{[ Salt ]}{[ Acid ]} \quad\left[\because p K_{a}=-\log K_{a}\right] \\
5=-\log \left[1 \times 10^{-4}\right]+\log \frac{[ Salt ]}{[ Acid ]} \\
5=4+\log \frac{[ Salt ]}{[ Acid ]} \\
5-4=\log \frac{\text { [Salt] }}{[ Acid ]} \\
1=\log \frac{[ Salt ]}{[ Acid ]} \\
\frac{[ Salt ]}{[ Acid ]}=10 \text { i.e., } 10: 1
\end{array}$
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Concepts Used:
Buffer solutions
The buffer solution is a solution that is able to maintain its Hydrogen ion concentration (pH) with only minor changes on the dilution or addition of a small amount of either acid or base. They are used in fermentation, food preservatives, drug delivery, printing, the activity of enzymes and many more.
Buffer solutions are aqueous solutions containing a weak acid and its salt - acid buffer or a weak base and its salt - base buffer.
Types of Buffer Solution
Acidic Buffers
These solutions are used to maintain acidic environments. Also, it has acidic pH and is prepared by mixing a weak acid and its salt with a strong base. An aqueous solution of an equal concentration of acetic acid and sodium acetate has a pH of 4.74.
Alkaline Buffers
These solutions are used to maintain basic conditions. It has a basic pH and is prepared by mixing a weak base and its salt with strong acid. The aqueous solution of an equal concentration of ammonium hydroxide and ammonium chloride has a pH of 9.25.