Question

I am supposed to make a 100ml buffer with a pH of 3.900 and a total common ion concentration of .180, using sodium acetate and acetic acid

pKa= 1.76x10-5

I began by using pH=pKa+log(base/acid)

And got the concentrations

Base=.0221

Acid=.1579

Is this correct? is this how it is supposed to be calculated or am I completely off?

Also from whatever the final concentrations should be, how do I go about creating a 100ml solution

Instructions say I have available 1 M of each sodium acetate and acetic acid, but I actually would have to make the sodium one from a solid.

Is using the base/acid concentrion x .1 L =moles x g/mol, for both and mix into a single 100 ml flask,A possibility

or should I simply make a 1 M solution of Of each then use m1v1=m2v2 to get the exact ml needed.

Answer 1

given total common ion conc is 0.18

here common ion is acetate

so

[acetic acid] + [sodium acetate ]= 0.18

now

consdier

pH = pKa + log [ base / acid ]

so

pH = -log Ka + log [ sodium acetate / acetic acid ]

3.9 = - log 1.76 x 10-5 + log [ sodium acetate / acetic acid ]

[ sodium acetate / acetic acid ] = 0.1398

so

[acetic acid ] = 7.15 x [sodium acetate ]

now

consdier

[acetic acid] + [sodium acetate ]= 0.18

this becomes

7.15 x [ sodium acetate ] + [ sodium acetate ] = 0.18

8.15 x [ sodium acetate ] = 0.18

[ sodium acetate ] = 0.0221 M

[acetic acid] = 0.18 - 0.0221= 0.1579 M

Yes your answers are correct

moles of acid in the solution does not change

only conc changes

so

the moles of acid/ base you added should be equal to the final moles of acid / base

use M1V1 = M2V2

for acetic acid

1 x V1 = 0.1579 x 100

V1 = 15.79 ml

so

take 15.79 ml of 1 M acetic acid

now

for sodium acetate

1 x V1 = 0.0221 x 100

V1 = 2.21 ml

so

take 2.21 ml of 1 M sodium acetate

after taking the above volumes

add water to make the final volume to 100 ml