[hahahanna]

Hi!
I have 2 questions about making buffers:

Im going to calculate a buffert with benzoic acid and NaOH.

1. first i want to calculate the buffer pH, and i have dine like this:

V_tot= 200*10^-3ml
m_benzoic=1,32 g
Mw_benzoic=122g/mol
V_NaOH= 24*10^-3 ml
C_NaOH= 1 M

I have calculate the concentration for benzoic acid and NaOH = C_{benzoic acid}=0,05 M and _NaOH=0,12M

I will use Henderson–Hasselbalch equation..
And the pKa for benzoic acid is 4,19.. So with the equation i will get the pH at the buffer: 6,59.

Have i thinking right now?

2. My other question is that i want to calculate pH in the same buffer above, only that i will take just 20 ml from it and then establish 1ml, 1M NaOH. What do i calculate with if i want to do that. and how? We can say that i want to take out 20 ml of my stocksolution and establish 1ml 1M NaOH to that.
Is it the concentration? But i which one do i use? The concentration for benzoic acid or NaOH???

please, i really need help.
And i have no book to look in.


thanks!

[Borek]

Please elaborate - you are dropping some numbers but you are not clearly stating what the question is.

[hahahanna]

My question is:
I want to do a buffer with benzoic acid and NaOH, and I will call this buffer when its finish for “Solution A”.
I want to have 200 ml of the buffer, and I have 1,32 g benzoic acid and 24 ml of 1,00 M NaOH.

C6H5COOH + OH-  C6H5COOH + H2O

1. first i want to calculate the buffer pH, and i have done like this:

Vtot= 200*10-3ml
mbenzoic=1,32 g
Mwbenzoic=122g/mol
nbenzoic= 0,01 mol

VNaOH= 24*10-3 ml
CNaOH= 1 M
nNaOH= 0,024 mol
Then I calculated the concentration in 200 ml:
CNaOH= n/V => 0,024/200*10-3 = 0,12 M
Cbenzoic= n/V => 0,01/200*10-3= 0,05 M
Now I will use Henderson–Hasselbalch equation..
And the pKa for benzoic acid is 4,19.. So with the Henderson–Hasselbalch equation i will get the pH at the buffer: 6,59.

Have i thinking right now?

2. My other question is that i want to calculate pH if I take 20 ml “Solution A” (the solution above) and establish 1ml, 1M NaOH. How do I calculate this pH?
Is it the concentration for benzoic acid that I use for the calculation?