Answer to Question #150843 in General Chemistry for Sarah

Question

Acetic acid, CH 3COOH (vinegar) is considered a weak acid because when dissolved in water

Answer 1

Q150843

Acetic acid, CH 3COOH (vinegar) is considered a weak acid because when dissolved in water

Solution:

An Acid is considered a strong acid if it dissociates completely in the water to give H⁺ ions.

A weak acid is the one that does not dissociate completely in the aqueous solution.  

Acetic acid dissociates partially ( to a very small extent) in water.

So not all acetic acid molecules dissociate in water.

Example: Consider a 0.1M HCl and 0.1M CH₃COOH ;

pH of 0.1M HCl will be

pH = -log [H+] = -log [ 0.1] = 1 ;

the pH of 0.1M CH₃COOH will be

$CH_3COOH + H_2 O(l)\space <---> \space CH_3COO^{-}(aq) + H_3O^{+}$

0.1 M ............................................................0 .....................................0............Initial

-x ........................................................................+x....................................+x..........change

0.1-x ...................................................................x ........................................x .......... Equilibrium

$K_a= \frac{[ CH_3COO^{-}] * [H_3O^{+}]}{[CH_3COOH]};$

dissociation constant Ka of acetic acid is 1.8 * 10^-5 ;

1.8*10^-5 = $\frac{x^2}{0.1-x};$

solving this equation for 'x' we have

x = 1.333 *10^-3 ;

[H+] = x = 1.333 *10^-3 ;

hence pH of 0.1M CH₃COOH is

pH = -log [ 1.333 *10^-3 ] = 2.88;

You can see that even when the concentration of HCl and CH₃COOH were the same, the pH of acetic

acid is much higher than HCl.

[ Lower pH means strong acid and Higher pH means weak acid.]

Acetic acid, CH₃COOH (vinegar) is considered a weak acid because when dissolved in water

it dissociates partially in an aqueous solution to give H⁺.