In the previous post, we talked about pH and its measure for acidity.

Remember, pH is defined as the negative logarithm of proton/hydronium ion concentration:

 

 

So, how do we know the concentration of proton or hydronium ion from the molarity of an acid?

Well, there are a few possibilities here depending on whether it is a strong or a weak acid. For strong acids such as HCl, HI, HClO₄, HNO₃, H₂SO₄ and etc, it is a relatively simple task because, for a strong acid, we assume a 100% dissociation, and therefore, the [H⁺] is the same as [HA].

Example: Calculate the pH of 2.49 M solution of HNO₃.

HNO₃ is completely ionized in water producing protons in a 1:1 molar ratio:

 

HNO₃(aq) → H⁺(aq) +NO₃^–(aq)

 

Therefore, [H⁺] = [HNO₃] = 2.49 M, and

 

pH = -log 2.49 = 0.396

 

The pH of a Strong Base

It is possible to calculate the pH from the concentration of a base.

For example, what is the pH of 0.450 M solution of NaOH?

The strategy here is to calculate the pOH and then, using the pH + pOH = 14 relationship, determine the pH.

NaOH is a strong base, so [OH^–] = [NaOH], and therefore, the pOH is:

 

pOH = -log 0.450 = 0.346

 

The pH then is equal to:

 

pH = 14 – pOH = 14 – 0.346 = 13.7

 

As expected, the pH is very high because it is a solution of a strong base.

In the next article, we will discuss determining the pH of a weak acid.

 

Check Also

• Definitions of Acids and Bases
• Acid-Base Reactions
• Acid-Base Titrations
• Conjugate Acid and Conjugate Base
• Autoionization of Water and K_w
• The pH and Acidity
• Acid Strength, K_a, and pK_a
• Base Strength, K_b and pK_b
• K_a, pK_a, K_b, and pK_b Relationship
• pH + pOH = 14
• The pH of a Weak Acid
• The pH of a Weak Base
• ThepH of Polyprotic Acids
• The acidity of a Salt Solution
• The pH of a Salt Solution
• The pH of Salts With Acidic Cations and Basic Anions
• Acids and Bases Practice Problems