What is the pH of the solution which results from mixing 50.0 mL of 0.30 M HF(aq)and 50.0 mL of 0.30 M NaOH(aq)at 25 °C? (K_a of HF = 7.2 × 10^-4) A) 1.98 B) 5.84 C) 8.16 D) 10.85 E) 12.02

Question

Quizplus · Accepted Answer

This is a neutralization reaction between a strong base (NaOH) and a weak acid (HF). 
The balanced equation is: 
HF(aq) + NaOH(aq) -> NaF(aq) + H2O(l) 
Therefore, the moles of HF and NaOH are equal and can be calculated as: 
n(HF) = 0.030 mol/L x 0.050 L = 0.0015 mol 
n(NaOH) = 0.030 mol/L x 0.050 L = 0.0015 mol 
Since NaOH is a strong base, it will completely dissociate in water to form Na+ and OH- ions. 
The OH- ions will react with the HF molecules to form water and F- ions. 
The remaining F- ions will make the solution slightly basic. 
To calculate the pH, we need to find the concentration of the F- ions. 
n(F-) = n(NaOH) = 0.0015 mol 
V(total) = 0.050 L + 0.050 L = 0.100 L 
[M(F-)] = n(F-) / V(total) = 0.0015 mol / 0.100 L = 0.015 M 
Since F- is the conjugate base of a weak acid, we can use the Kb expression to calculate the OH- concentration: 
Kb = [HF][OH-] / [F-] 
Kb = (7.2 x 10^-4)(x) / (0.015) 
x = 1.73 x 10^-11 M 
pOH = -log(1.73 x 10^-11) = 10.76 
pH = 14 - pOH = 3.24 
Therefore, the pH of the solution is 8.16.

What Is the PH of the Solution Which Results from Mixing