The Ph Scale

The pH scale (potency of hydrogen) – A measurement of acid (and indirectly, base strength) The pH scale is a logarithmic scale that has values (mostly) between 0 and 14. (It can go less than zero and above 14) - 7 is neutral (this is where the [H+] = [OH-] ) - As the H3O+ ion concentration rises the OH- concentration falls and vice versa. See below. - The scale rates the concentration of the H+ or more accurately the H3O+ ion. (H+ is called a proton, the H3O+ is called a hydronium or oxonium or hydroxonium ion) - The higher the concentration of the H3O+ the stronger the acid and simultaneously, the weaker the base as the OH- concentration falls. - pH = -log [H3O+ ] - Any value below 7 is considered acidic - Any value above 7 is considered basic - The scale is a continuum. If a solution is a strong acid it is simultaneously a very weak base. The opposite is also true. A strong base (pH =14ish) is also a very weak acid.

Acidic

(neutral) 7 + [H3O ] = [OH-] = 1.0 x 10-7

0 + [H3O ] = 1.0 Molar [OH-] = 1.0 x 10-14

Basic 14 +

-14

[H3O ] = 1.0 x 10 M [OH-] = 1.0 M

** Notice: When the [H3O+] is high, the [OH-] is low and vice versa. They are two sides of the same coin!!

How do we use the pH scale? If you are given, or are able to calculate, the concentration of the hydronium ion, then calculating the pH is easy. Simply: pH= -log [H3O+ ] Example: Q. You are given a .25 M solution of HCl (aq). What is its pH? A. HCl is only able to donate one proton to water as follows: HCl (g)

+

H2O (l)



H3O+(aq)

+

Cl- (aq)

Ka = ?

As can be seen from the balanced chemical reaction, the HCl only donates one proton. Therefore the concentration of the H3O+ is the same as the HCl (aq) solution. Therfore [H3O+] = .25 M. Therefore: pH = -log [H3O+ ] = -log [.25 M] =.60206 The 0.6206 is very close to zero. In other words an acidic solution. Q A.

You might ask why is it important to write the balanced chemical equation?? Because many acids can donate more than on proton. Check this out! Q. A.

What is the pH of a 0.25 M solution of H2SO4(aq) (sulphuric acid!) H2SO4 can donate two protons to water as follows:

H2SO4 (g) + 2H2O (l)  2H3O+(aq) + SO4-2 (aq) Ka = ? As can be seen from the balanced chemical equation. Each produces two (!) hydronium ions. Therefore the concentration of the H3O+(aq) is DOUBLE the concentration of the solution. Therefore [H3O+(aq)] = (0.25) x 2 = 0.50 M Therefore: pH = -log [H3O+(aq)] = -log [0.50 M] = .30103 The .30103 is even closer to zero. (In fact half the above!) Can you explain why ??

What is a Ka? What does it tell us about the strength of an acid?