Determine the pH of a buffer made from 250 ml of 0.50M HF and 150 ml of 0.75 M NaF.
What's the problem. Use the HH equation. If you want me to check it let me know what you're using for pKa for HF.
I'm not sure where to start. Is this correct so far?
I first write a balanced chemical equation using HF and water
HF+ H2O <->H3O^+ + F^-
The use the ICE table where the E row is:
HF=0.50-x
H30+= x
F-= 0.75+x
Then the Ka equation
Ka=[H3O+][A-]/[HA] where Ka=(x)(0.75+x)/0.50-x
You are making it far too complicated.
pH = pKa + log (base)/(acid)
millimols base = 150 x 0.75 = ?
millimols acid = 250 x 0.50 = ?
Substitute the millimols into acid/base and solve for pH.
Tehcnically, one can't substitute millimols; i.e., it is supposed to be concentration base and concentration acid BUT note that M (which is the concentration) = millimoles/mL and since the numerator and denominator BOTH have the same mL, the volume cancels and you are left with millimoles. Therefore, millimoles works just fine and that saves the two steps of calculating the concentrations of each.
To determine the pH of a buffer, we need to use the Henderson-Hasselbalch equation. The Henderson-Hasselbalch equation is pH = pKa + log ([A-]/[HA]), where pKa is the negative logarithm of the acid dissociation constant, [A-] is the concentration of the conjugate base, and [HA] is the concentration of the acid.
In this case, HF is a weak acid and NaF is its conjugate base. The pKa of HF is known to be 3.17.
Step 1: Calculate the concentrations of acid and base in the buffer solution.
Given:
Volume of HF (acid) = 250 ml = 0.25 L
Concentration of HF (acid) = 0.50 M
Volume of NaF (base) = 150 ml = 0.15 L
Concentration of NaF (base) = 0.75 M
To calculate the amount of acid and base, we need to use the formula:
Amount (mol) = concentration (M) x volume (L)
Amount of HF (acid) = 0.50 M x 0.25 L = 0.125 mol
Amount of NaF (base) = 0.75 M x 0.15 L = 0.1125 mol
Step 2: Calculate the ratio of [A-]/[HA].
[A-]/[HA] = Amount of NaF / Amount of HF
= 0.1125 mol / 0.125 mol
= 0.9
Step 3: Use the Henderson-Hasselbalch equation to calculate the pH.
pH = pKa + log ([A-]/[HA])
= 3.17 + log (0.9)
= 3.17 + (-0.0458)
= 3.1242
Therefore, the pH of the buffer solution is approximately 3.12.