Answer:
Explanation:
This can be contradictory, depending on whether the 0.1 M
is the total species concentration or the concentration of each of the two components. I'll consider this to be the former...
VA− = 9.125 mL
VHA = 15.875 mL
The Henderson-Hasselbalch equation is:
pH = pKa + log [A−][HA]
We have a pH 4.5
solution of acetic acid and acetate, so from there we can get the ratio of weak acid to conjugate base:
[A−][HA]=10
pH − pKa = 104.5 − 4.74 = 0.5754
Now, if the total concentration is
0.10 M , then:
[HA] + [A−] 0.5754
[HA] = 0.10 M
⇒[HA] = 0.10 M 1.0000 +0.5754
= 0.0635 M
−−−−−−−−
⇒[A−] = 0.0365 M
−−−−−−−−
and these concentrations are AFTER mixing. Since the total volume is 50 mL , or 0.050 L, the mols of each component (which are constant!) are:
nA − = 0.0365 molL × 0.050L =
0.001825 mols
−−−−−−−−−−−−
nHA = 0.0635 molL × 0.050L =
0.003175 mols
−−−−−−−−−−−−
So, if both of the starting concentrations were
0.20 M, we can find the volume they each start with:
VA − = 1 L0.20mols
A− × 0.001825mols A− = 0.009125 L = 9.125 mL
−−−−−−−−
VHA = 1 L 0.20 mols HA × 0.003175
mols HA = 0.015875 L = 15.875 mL
−−−−−−−−−
And this should make sense, because the total starting volume is
25.000 mL , the total ending volume is twice as large; the total species concentration is half the concentration that both species started with.
