Find the pH of a 0.25 M solution of lactic acid, HC3H5O3 (Ka = 8.3 x 10 -4 ).

pH → 1.83

Explanation:

Let's write the lactic acid as HLac, because it is a monoprotic weak acid.

HLac + H₂O  ⇄  H₃O⁺  +  Lac⁻      Ka

Initially we have 0.25 moles, so x amount has reacted. At the end, when the equilibrium is finished, we may have (0.25 - x) moles of acid, x moles of protons have been released and x moles of lactate were formed.

In order to find x, we use the acid constant, Ka. The expression for Ka is:

Ka = [H₃O⁺] .  [Lac⁻] / [HLac]

8.3×10⁻⁴  = x² / (0.25-x)

8.3×10⁻⁴ (0.25 - x) = x²

8.3×10⁻⁴ . 0.25 - 8.3×10⁻⁴x - x² → this is a quadractic function

a = -1,  b = 8.3×10⁻⁴, c = 2.07×10⁻⁴

We solve: (-b - √(b² - 4ac) / (2a)

(-8.3×10⁻⁴ - (√ ((8.3×10⁻⁴)² - 4 (-1) (2.07×10⁻⁴) ) / (2 . -1) = 0.0148

[H₃O⁺] = 0.0148 M

- log [H₃O⁺] = pH → - log 0.0148 = 1.83

one negative thing it that it can kill water animals. another negative thing is it can cause the destruction of ecosystem. one way water pollution can be reduced is to conserve energy.