Henderson-Hasselbalch and PH buffer equation

[ahhppull]

Homework Statement

Calculate the ph of a buffer made by mixing 50.0 mL of 0.10M of acetic acid with 50.0 mL of 0.10M sodium acetate. Use the Henderson-Hasselbalch equation.

Homework Equations

pH = pKa + log ([base]/[acid])

The Attempt at a Solution

I know how to do this problem if the 50.0 mL information isn't included. Does the volume even matter?

Heres my work without including the 50.0 mL
ICE TABLE
HC2H3O2 H3O+ C2H3O-
I .100 M 0.00 M .100M
C -x +x +x
E .100 - x x .100 + x

I get a concentration of 1.8E-5 M and pH will be 4.74

 

[sjb-2812]

Homework Statement

Calculate the ph of a buffer made by mixing 50.0 mL of 0.10M of acetic acid with 50.0 mL of 0.10M sodium acetate. Use the Henderson-Hasselbalch equation.

Homework Equations

pH = pKa + log ([base]/[acid])

The Attempt at a Solution

I know how to do this problem if the 50.0 mL information isn't included. Does the volume even matter?

Heres my work without including the 50.0 mL
ICE TABLE
HC2H3O2 H3O+ C2H3O-
I .100 M 0.00 M .100M
C -x +x +x
E .100 - x x .100 + x

I get a concentration of 1.8E-5 M and pH will be 4.74

What do the square brackets, [ and ] indicate?

 

[ahhppull]

What do the square brackets, [ and ] indicate?

Concentration in molarity

 

[Borek]

So, you have 50mL of 0.1M aetic acid solution, you mix it with 50mL of other solution - what is new concentration of acetic acid?

And why do you use ICE table after stating pH should be calculated using Henderson-Hasselbalch equation?

 

[ahhppull]

.05 M? Do I put this for E of acetic acid?

 

[ahhppull]

I still need some help. I've been thinking and I think that I don't have to use an ICE chart. I think that using the henderson-hasselbalch equation is only needed.

pH = pKa + log ([base]/[acid])
pH = 4.74 + log [?]/[?]

(4.74 is from the -log(Ka of acetic acid)
I don't understand how to find the concentration of the base and the acid. I don't really understand buffers, but if you add the same amounts of acetic acid and sodium acetate, wouldn't the rate of dissociation be the same?

 

[Borek]

I don't understand how to find the concentration of the base and the acid. I don't really understand buffers, but if you add the same amounts of acetic acid and sodium acetate, wouldn't the rate of dissociation be the same?

Find concentrations just from dilution. Actually you can even use just number of moles of each substance, as the final volume is the same for both and it cancels out.

Your problem here (and understandable one) is that you want to calculate new equilibrium, assuming some acid dissociated and some conjugate bases hydrolized - while this is true, these effects in most cases are almost identical, so they cancel out and you can safely ignore them. Please read Henderson-Hasselbalch equation page that I linked to earlier.