A solution with a pH of 2.0 is how many times more acidic than than a solution with a pH of 4.0?

Question

A solution with a pH of 2.0 is how many times more acidic than a solution with a pH of 4,0?

Answer 1

Here is a way to look at it.  Since by definition,

 

pH = -log[H₃O⁺],

 

[H₃O⁺] = 10^-pH

 

If pH = 2, then [H₃O⁺] = 10^-2

 

If pH = 4, then [H₃O⁺] = 10^-4

 

Since 10^-2 = (100)10^-4, the concentration of [H₃O⁺] is 100 times as great at pH = 2 than at pH = 4, so the acid is 100 times as strong at pH = 2 than at pH = 4.

 

As Sanjay explained, the pH scale is logarithmic to base 10, so a change of one unit in pH corresponds to a change by a factor of 10 in the concentration of H₃O⁺.

Answer 2

For a difference of one pH unit the hydrogen ion concentration changes 10 fold. This is because pH is measured as the negative log of H2 ion concentration, making one pH unit 10 fold different at H2 ion concentration.