In this lesson, you’ll learn how hydronium ions are made and why they form in solution. You’ll also learn the formulas for calculating hydronium ions by finding the pH.

Water & Hydronium Ions

Leonardo Da Vinci is credited with saying, ‘Water is the driving force of all nature.’ Water composes most of your body, and the planet earth. For something as prevalent and necessary to life as water is, it must be a remarkable substance!

A single water molecule is made up of two hydrogen atoms that are bonded to one oxygen atom. It’s chemically written as H2O. By themselves atoms are small units, but when combined together to make molecules, they compose everything in the universe. Atoms can be likened to letters of the alphabet, and the infinite amounts of words that can be made from only a few letters. Such is the diversity of molecules.

Two atoms, such as hydrogen and oxygen for example, are found in almost every substance on earth. It is the interactions between these two atoms in water that yield a special type of molecule called a hydronium ion.

Hydronium ions are water molecules that have gained an extra positive hydrogen ion. In this lesson, we’ll explore how hydrogen can become a positive ion, how water can become a hydronium ion, and how to calculate the concentration of hydronium ions in solution.

Origins of an Ion

All atoms are composed of three subunits:

  1. Electrons, which have a negative charge.
  2. Protons, which have a positive charge.
  3. Neutrons, which as their name implies, are neutral.

The ratio of these three subunits contributes to the property of the atom. Atoms bond together to form molecules, but atoms can break away from the molecules as well. When the atoms break from their bonds, electrons can shift away and stay behind.

This creates an ion, which is an atom or molecule that has lost or gained an electron. Hydrogen is only made of one electron and one proton. Sometimes, when hydrogen breaks its bond with a molecule, its electron stays with the molecule. When the hydrogen loses its electron it becomes an ion, although it’s just a single proton. Because of the proton, it has an overall positive charge, written as H+.

Hydronium ; Hydroxide Ions

A water molecule, also written as H-O-H, can become unstable over time, and when it does, one hydrogen atom separates from H-O-H and becomes H+. Sometimes, if another molecule like hydrogen chloride (HCl) is dissolved in water, the hydrogen will dissociate as H+.

In an aqueous solution, the added H+ becomes attracted to the negative poles on another water molecule. This leaves an H2O molecule with an extra hydrogen atom, written as H3O, called a hydronium ion.

If a solution has a ton of hydronium ions it becomes acidic. The concentration of hydronium ions directly relates to pH, which we’ll discuss in the next part of the lesson.

In the mean time, what happened to the original water molecule abandoned by the hydrogen? Hydroxide ions are what’s left of water when a hydrogen ion breaks away. Hydroxide ions are written as OH- and have an overall negative charge, because they steal other atoms’ electrons. Hydroxide ions can also enter solutions also when they disassociate from other molecules. When dissolved in water, NaOH, or sodium hydroxide, becomes a sodium ion and a hydroxide ion.

In pure water, these two ions are equal in concentration and have a neutral, or 7, pH. In neutral solutions the concentration of each is 10-7 molarity. So the combined concentration of both is 10-14. If hydronium ions increase in number, they will bond with hydroxide to produce water again.

The reverse is also true. If hydroxide ions increase in number, they’ll bond with hydronium ions to produce water. Therefore, the concentration of both hydronium and hydroxide ions will never exceed 10-14 molar concentration. The product of both the hydronium and hydroxide ions will always equal 10-14.

[OH-] x [H3O+] = 10-14

By knowing the concentration of one ion in solution, you can deduce the concentration of the other. For example if the concentration of H3O+ is 10-8, you can find the concentration of OH-.

[OH-] x [10-8] = 10-14

[OH-] = 10-6

Formulas

To calculate how many hydronium ions are in solution, you must find the pH. Simple probes dipped in solution can give you a number from 0 to 14. A reading below 7 pH is an acidic solution; a reading above 7 pH is a basic solution. These numbers refer to the concentration of hydronium ions in that solution. pH is the negative log of the concentration of H3O+. The formula would be written as:

pH = – [log H3O+].

Another way to think of this is [H3O+ ] = 10-pH.

Let’s look at an example. Say you’re testing a water sample from the Detroit River. You find that the pH is 5.5. Using the formula [H3O+]=10-pH, you find there are 10-5.5 molar concentrations of H3O+ in that sample. Knowing pH directly helps you find the concentration of hydronium ions.

Lesson Summary

In this lesson, you learned that hydrogen can become a positive ion when it breaks its bond in a molecule and loses its electron. Water becomes a hydronium ion (H3O+) when a positive hydrogen ion attracts to the negative pole of another water molecule. Hydroxide ions (OH-) are what is left of water when a hydrogen ion breaks away.

You can calculate the concentration of hydronium ions in solution if you know the concentration of hydroxide ions by using the formula [OH-] x [H3O+] = 10-14. You can also calculate the concentration of hydronium ions if the pH, or negative log of the concentration of H30+, is known by using the formula [H3O+] = 10-pH.

Three useful formulas for calculating hydronium ions in solution are:

  • [OH-] x [H3O+] = 10-14
  • [H3O+] = 10-pH
  • pH = – [log H3O+]