pH value and acidity

[aman]

If the pH value of a solution is 4. Then acidity is 14 - 4 = 10. Define acidity and why we have to subtract 4 from 14 to get the acidity?

[Russ]

Unless this is somehow beyond me, I'm going to move this from this section.

My answer would be something along the lines of;

pH is calculated by the formula

pH = log [H+]

Concentration of H+ can be calculated from the formula

[H+][OH-] = 10^14 (at 25 degrees)

Thus, if the pH is 4, the concentration of H+ ions is 10^4 and thus the concentration of OH- ions is 10^10. That's where the "subtract X from 14" comes from.