Strong base is dissolved in 665 ml. of 0.200 M weak acid (K, 4.73 x 10) to make a buffer with a pH of 4.10. Assume that the volume remains constant when the base is added.

Question

HA(aq) + OH (aq) → H₂O(I)+A(aq)

Calculate the pK, value of the acid, and determine the number of moles of acid initially present.

Answer 1

To solve this problem, we will first calculate the pKa of the weak acid using the provided \( K_a \) value and then determine the number of moles of the acid initially present.

Step 1: Calculate pKa

The \( K_a \) value given for the weak acid is \( 4.73 \times 10^{-5} \).

To find \( pK_a \): \[ pK_a = -\log(K_a) \] Calculating \( pK_a \): \[ pK_a = -\log(4.73 \times 10^{-5}) \approx 4.32 \]

Step 2: Determine the number of moles of the weak acid initially present

The concentration of the weak acid (HA) is given as \( 0.200 , M \), and the volume is \( 665 , ml \) (or \( 0.665 , L \)).

To find the number of moles, use the formula: \[ \text{moles} = \text{concentration} \times \text{volume} \] Calculating the number of moles of HA: \[ \text{moles of HA} = 0.200 , M \times 0.665 , L = 0.133 , \text{moles} \]

Summary of Results

1. pKa of the weak acid: \( pK_a \approx 4.32 \)
2. Moles of the weak acid initially present: \( 0.133 , \text{moles} \)

These are the required calculations based on the information provided.