Question

Which statement best describes the pH of pure water?

Answer 1

Answer : The correct option is, It is neutral because the concentration of hydronium ions equals that of hydroxide ions.

Explanation

Dissociation constant of pure water : It is defined as the product of the concentration of hydrogen ion and hydroxide ion.

The equilibrium dissociation reaction for water will be:

$H_2O(aq)\rightleftharpoons H^+(aq)+OH^-(aq)$

The expression for dissociation constant for water will be:

$k_w=[H^+][OH^-]$

Experimentally, at $25^oC$ temperature, the dissociation constant of pure water is found to be $1.0\times 10^{-14}mol/L$.

As we know that pure water is a neutral solution because the concentration of hydrogen ion is equal to the concentration of hydroxide ions.

That means,

$k_w=[H^+][OH^-]$

$k_w=[H^+][H^+]$

$k_w=[H^+]^2$

$1.0\times 10^{-14}=[H^+]^2$

$[H^+]=1.0\times 10^{-7}mol/L$

As we know that, pH is the negative logarithm of hydrogen ion concentration.

$pH=-\log [H^+]$

$pH=-\log (1.0\times 10^{-7})$

$pH=7$

We know that the pH range is, 1 to 14.

When the value of pH is less than 7 then the solution is acidic.

When the value of pH is more than 7 then the solution is basic.

When the value of pH is equal to 7 then the solution is neutral.

Hence, the best statement for the pH of pure water is, It is neutral because the concentration of hydronium ions equals that of hydroxide ions.

Answer 2

The statement which best describes the pH of pure water is the first one among the choices. the pH of pure water is neutral because the concentration of hydronium ions equals that of hydroxide ions. I hope this helps.