10. The pH of an aqueous solution changes from 4 to 3 when the hydrogen ion concentration in the solution is

A)    Decreased by a factor of 4/3

B)    Decreased by a factor of 10

C)    Increased by a factor of 3/4

D)    Increased by a factor of 10

Correct Answer: Option D - Increased by a factor of 10

pH is a measure of the acidity or basicity of a solution. It is defined as the cologarithm of the activity of dissolved hydrogen ions (H+). Hydrogen ion activity coefficients cannot be measured experimentally, so they are based on theoretical calculations. The pH scale is not an absolute scale; it is relative to a set of standard solutions whose pH is established by international agreement.

Pure water is said to be neutral. The pH for pure water at 25 °C (77 °F) is close to 7.0. Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are said to be basic or alkaline.

pH is defined as minus the decimal logarithm of the hydrogen ion activity in an aqueous solution. By virtue of its logarithmic nature, pH is a dimensionless quantity.

where a_H is the (dimensionless) activity of hydrogen ions. The reason for this definition is that a_H is a property of a single ion which can only be measured experimentally by means of an ion-selective electrode which responds, according to the Nernst equation, to hydrogen ion activity.

Hence, each time you go down 1 on the pH scale, the concentration of hydrogen/ hydronium ions increases by 10.

Hence, options A, B and C are incorrect.