Question

What is [H₃O⁺] in a solution of 0.25 M CH₃CO₂H and 0.030 M NaCH₃CO₂?

a) 1.8 × 10⁻⁵ M
b) 1.5 × 10⁻⁴ M
c) 2.5 × 10⁻⁴ M
d) 3.0 × 10⁻³ M

Answer 1

The concentration of hydronium ions [H3O+] in a buffer solution of 0.25 M CH3CO2H and 0.030 M NaCH3CO2 is calculated to be approximately 3.8 × 10^-4 M, which is not one of the given options.

Step-by-step explanation:

The calculation of the concentration of hydronium ions [H3O+] in a solution containing acetic acid (CH3CO2H) and sodium acetate (NaCH3CO2) involves using the Henderson-Hasselbalch equation, as this is a buffer solution. For acetic acid, the equilibrium constant (Ka) is commonly given as 1.8 × 10-5.

The Henderson-Hasselbalch equation is defined as:

pH = pKa + log([A-]/[HA])

Where [A-] is the concentration of the acetate ion and [HA] is the concentration of acetic acid. In this scenario:

• [HA] = 0.25 M
• [A-] = 0.030 M

First, we calculate the pH:

pH = -log(1.8 × 10-5) + log(0.030/0.25)

pH = 4.74 + (-1.322)

pH = 3.418

Now, we convert the pH to [H3O+]:

[H3O+] = 10-pH

[H3O+] = 10-3.418

[H3O+] = 3.8 × 10-4 M

Thus, the concentration of hydronium ions in the buffer solution is approximately 3.8 × 10-4 M, none of the provided options accurately represent this value.