Answer to Question #268746 in Chemistry for eli

Question

The K_b of ammonia is 1.76 × 10^-5. The pH of a buffer prepared by combining 45.0 mL of 0.180 M ammonia and 40.0 mL of 0.205 M ammonium nitrate is __________.

Answer 1

Solution:

ammonia = NH₃

ammonium nitrate = NH₄NO₃

Recalculate the molar concentration of each species:

V_Solution = V_NH3 + V_NH4NO3 = 45.0 mL + 40.0 mL = 85.0 mL

C_M(NH₃) = C_o(NH₃) × (V_NH3 / V_Solution) = (0.180 M) × (45.0 mL / 85.0 mL) = 0.0953 M

[NH₃] = 0.0953 M

C_M(NH₄NO₃) = C_o(NH₄NO₃) × (V_NH4NO3 / V_Solution) = (0.205 M) × (40.0 mL / 85.0 mL) = 0.0965 M

[NH₄⁺] = 0.0965 M

To calculate the pH of a given buffer, you need to use the Henderson-Hasselbalch equation for basic buffers: pH = 14 - (pK_b + log([B⁺]/[BOH])

[B⁺] = [NH₄⁺] = 0.0965 M

[BOH] = [NH₃] = 0.0953 M

pK_b = -log(K_b) = -log(1.76×10^-5) = 4.75

Hence,

pH = 14 - (4.75 + log(0.0965 / 0.0953) = 9.24

pH = 9.24

Answer: 9.24