# Question f4d64

May 25, 2017

Basic

#### Explanation:

For any solution at ${25}^{o} C$, the product of the $\left[{\text{OH}}^{-}\right]$ and $\left[{\text{H"_3"O}}^{+}\right]$ always equals a defined constant:

K_"w" = ["OH"^-]["H"_3"O"^+] = 1.00 xx 10^-14 M^2

The constant ${K}_{\text{w}}$ is the equilibrium constant for water, called the ion-product constant, and is equal to $1.00 \times {10}^{-} 14 {M}^{2}$

From this relationship, we can see that $\left[{\text{OH}}^{-}\right]$ and $\left[{\text{H"_3"O}}^{+}\right]$ are inversely proportional; as the concentration of hydroxide ions increases, the concentration of hydronium/hydrogen ions decreases.

An acidic solution is one in which the hydronium ion concentration is greater than that of the hydroxide ion concentration. Why? Because the acidity of a solution can be measured by the solution's $\text{pH}$; a lower $\text{pH}$ indicates a higher $\left[{\text{H"_3"O}}^{+}\right]$. If the $\text{pH} < 7$, the solution is said to be acidic, and $\left[{\text{OH"^-] < ["H"_3"O}}^{+}\right]$. For a neutral solution, $\text{pH} = 7$, and the hydronium and hydroxide concentrations are equal. If the $\text{pH}$ is higher than $7$, the solution is basic, and because of the equation

K_"w" = ["OH"^-]["H"_3"O"^+] = 1.00 xx 10^-14 M^2#

the $\left[{\text{OH}}^{-}\right]$ is larger than the $\left[{\text{H"_3"O}}^{+}\right]$. Thus, if $\left[{\text{OH"^-] > ["H"_3"O}}^{+}\right]$, the solution is basic ($\text{pH} > 7$).