Explain how the concentration of hydronium ions and hydroxide ions are related to pH.
The concentration of hydronium ions (H3O+) and hydroxide ions (OH-) are directly related to pH, which is a measure of the acidity or alkalinity of a solution. The pH scale ranges from 0 to 14, with 7 being neutral, values below 7 being acidic, and values above 7 being alkaline.
To understand the relationship, we need to introduce the concept of the pH equation, which is pH = -log[H3O+]. This equation tells us that pH is determined by the logarithm of the concentration of hydronium ions.
In pure water, the concentration of hydronium ions is equal to the concentration of hydroxide ions. This is because water molecules can react with each other to form both H3O+ and OH- ions, following the equation H2O --> H3O+ + OH-. At room temperature, the concentration of each of these ions is 1 x 10^-7 moles per liter. Therefore, the pH of pure water is neutral, with a value of 7.
In an acidic solution, the concentration of hydronium ions is greater than the concentration of hydroxide ions. This results in a lower pH value, below 7. The higher the concentration of H3O+ ions, the lower the pH value, indicating a stronger acid.
Conversely, in an alkaline (or basic) solution, the concentration of hydroxide ions is greater than the concentration of hydronium ions. This leads to a higher pH value, above 7. The higher the concentration of OH- ions, the higher the pH value, indicating a stronger base.
So, in summary, the concentration of hydronium ions and hydroxide ions in a solution determines its pH. The balance between these ions determines whether a solution is acidic, alkaline, or neutral on the pH scale.