1) Calculate the pH and find the fraction of dissociation of 0.5 M benzoic acid.

Question

1) Calculate the pH and find the fraction of dissociation of 0.5 M benzoic acid. 2) Find the pH and the fraction of association of a 0.02 M 4-methoxyaniline solution. 3) Find the pH of a 3.0 M sodium acetate (CH3COONa) solution. 4) a) Calculate the molar ratio of formic acid (HCOOH)/sodium formate (HCOONa) to prepare a pH-5.50 buffer solution. b) If you have 1.000 g of formic acid dissolved in 1 L of water, how many grams of sodium formate do you need to add to this solution to prepare the pH-5.50 buffer? Calculate the pH of a buffer solution made from 1.000 g of sodium iodate NalOs and 0.523 g of iodic acid HIO3 dissolved in 1 L of water? 5) 6) What is a buffer capacity?

Explanation / Answer

Q1.

First, assume the acid:

Hbenzoic

to be HA, for simplicity, so it will ionize as follows:

HA <-> H+ + A-

where, H+ is the proton and A- the conjugate base, HA is molecular acid

Ka = [H+][A-]/[HA]; by definition

initially

[H+] = 0

[A-] = 0

[HA] = M;

the change

initially

[H+] = + x

[A-] = + x

[HA] = - x

in equilbrirum

[H+] = 0 + x

[A-] = 0 + x

[HA] = M - x

substitute in Ka

Ka = [H+][A-]/[HA]

Ka = x*x/(M-x)

x^2 + Kax - M*Ka = 0

if M = 0.5 M; then

x^2 + (10^-4.2)x - 0.5 *(10^-4.2) = 0

solve for x

x =0.005585

substitute

[H+] = 0 + 0.005585= 0.005585 M

[A-] = 0 + 0.005585= 0.005585 M

pH = -log(H+) = -log(0.005585 ) = 2.252

For ionization

% ion = [A-]/[HA]0*100% = 0.005585 /0.5 * 100 = 1.117%

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