a) Write the formula for each component in a buffer solution of acetic acid and its’ salt.
b) The Ka, for hypochlorous acid, HClO, is 7.2 x l0^–4 at 25°C. What is pKa? Write the equation for the reaction to which Ka applies.
CH3COOH and CH3COO^-
Ka = (H^+)(ClO^-)/(HClO)
pKa = -log Ka
a) To write the formula for each component in a buffer solution of acetic acid and its salt, we first need to understand how a buffer solution is formed. A buffer solution is made up of a weak acid and its conjugate base (salt) that help maintain a stable pH when small amounts of acid or base are added.
1. Acetic acid (weak acid): The formula for acetic acid is CH3COOH. It is a weak acid because it only partially dissociates into its ions in water.
2. Acetate ion (conjugate base/salt): The salt of acetic acid is formed by replacing a hydrogen ion (H+) from acetic acid with a cation. For example, if we use sodium as the cation, the salt of acetic acid is sodium acetate (CH3COONa).
The formula for a buffer solution containing acetic acid and sodium acetate is:
CH3COOH + CH3COONa
b) The pKa is the -log of the acid dissociation constant (Ka). To calculate the pKa, we need to take the negative logarithm (base 10) of the Ka value provided.
Given that the Ka for hypochlorous acid (HClO) is 7.2 x 10^–4 at 25°C, we can now calculate the pKa:
pKa = -log(Ka)
= -log(7.2 x 10^–4)
= -(-3.14)
= 3.14
Therefore, the pKa of hypochlorous acid is 3.14.
The equation for the reaction to which Ka applies is the dissociation of hypochlorous acid into its ions:
HClO ⇌ H+ + ClO-