At 20°c, the ion-product constant of water, kw, is 6.88 x 10-15 . what is the ph of pure water at 20°c?
a) 7.000
b) 6.501
c) 7.181
d) 7.081
e) none of these

The correct answer is D) 7.081

Explanation:

The water equilibrium is the following:

H₂O ⇄  H⁺ + OH⁻      Kw

Where Kw= [H⁺] x [OH⁻]

In pure water: [H⁺] = [OH⁻]= C

If we introduce C in the expression for Kw:

Kw= [H⁺] x [OH⁻]= C x C= C²

⇒ C= = = 8.29 x 10⁻⁸

pH= -log [H⁺] = -log C = -log (8.29 x 10⁻⁸) = 7.081

every chemical equation adheres to the law of conservation of mass, which states that matter cannot be created or destroyed. therefore, there must be the same number of atoms of each element on each side of a chemical equation.