Question

PART #1

Calculate [H30⁺][OH^-] for each of the following solutions;

two signifacant figures

pH =8.59 [H₃O] & [OH^-]

pH=11.20 [H₃O][OH^-]

pH= 2.94 [H₃O][OH^-]

Answer 1

The negative log of concentration of hydronium ions or protons in a medium is given as pH. Since hydronium ions are highly unstable species formed by the association of the strongly positive protons with the electronegative oxygens of water, pH is often taken as the concentration of protons themselves in the medium thus giving a measure of acidity or alkalinity. If the negative logarithm of concentration of hydroxide ions are to be calculated, the term pOH is used but only seldom. This is not found often as pH and pOH as related as pH + pOH = 14 thus giving all pOH as pH by pH = 14 - pOH and vice versa if required, making pH the sole analytical source for the measure of acidity or alkalinity of a medium.

From a given pH of a medium, the precise concentration of protons and/or hydroxide ions can be found as [H⁺] = 10^-pH and [OH-] = 10^-(14-pH) or 10^-pOH.

Thus,

a) For a pH of 8.59 concentration of protons will be 10^-8.59 = 2.57x10^-9 and concentration of hydroxide ions will be 10^-pOH = 10^-5.41 = 3.89x10^-6.

b) For a pH of 11.20, [H₃O⁺] = 10^-pH = 10^-11.20 = 6.31x10^-12 and [OH^-] = 10^-pOH = 10^-(14-11.2) = 10^-2.8 = 1.58x10^-3.

c) And for a pH of 2.94, [H₃O⁺] = 10^-pH = 10^-2.94 = 1.148x10^-3 and [OH^-] = 10^-pOH = 10^-(14-2.94) = 10^-11.06 = 8.71x10^-12.

The units of concentration of protons and hydroxide ions in the medium can be in terms of moles or molarity if the volume of solution is taken as 1L.