pH formula

General Chemistry

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.

 

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