What is the initial pH for the titration of 30.0 mL of 0.850 M NH₃ with 0.100 M HCl. K_b for NH₃ = 1.8 × 10⁻⁵.

• A. 2.40
• B. 4.14
• C. 4.66
• D. 9.34
• E. 11.60

Answer: (E)

Verified step by step guidance

1. Start by identifying the species involved in the solution. Here, we have ammonia (NH3), which is a weak base, and its concentration is given as 0.850 M.
2. To find the initial pH, we need to calculate the pOH first, since NH3 is a base. Use the formula for the base dissociation constant (Kb) to find the concentration of OH⁻ ions: \( K_b = \frac{[NH_4^+][OH^-]}{[NH_3]} \).
3. Assume that the initial concentration of OH⁻ is x. Therefore, \([NH_4^+] = x\) and \([OH^-] = x\). Substitute these into the Kb expression: \( 1.8 \times 10^{-5} = \frac{x^2}{0.850 - x} \).
4. Since Kb is small, we can assume \( x \ll 0.850 \), simplifying the expression to \( 1.8 \times 10^{-5} = \frac{x^2}{0.850} \). Solve for x to find the concentration of OH⁻ ions.
5. Convert the concentration of OH⁻ ions to pOH using the formula \( pOH = -\log[OH^-] \). Finally, use the relationship \( pH + pOH = 14 \) to find the initial pH of the solution.