The pH Cheat Sheet

The pH Cheat Sheet

BY : Rhiannon Nevinczenko


Terms to Know:

Ion: An atom with either a positive or negative charge, rather than neutral. For example, a cation is an atom that lost an electron, and now has a positive charge.

Logarithm: A mathematic function that describes the exponent to which a base (b) must be raised to produce a number (x). In other words: logb(bx) = x. For example, a pH of 3 is 10x more acidic than a pH of 4, ad 100x more acidic than a pH of 5.

Base (hydroxide solution): A substance that loses H+ ions. Any substance with a pH greater than 7 is a base.

Acid (hydronium solution): A substance that gains H+ ions. Any substance with a pH lower than 7 is an acid.

Equilibrium: A state wherein products and reactants are present in concentrations that balance out – a forward reaction is equal to a backward reaction (reversible chemical reaction).


What is pH, anyway?

We measure acidity and basicity (a.k.a. alkalinity) using pH. This is done using a logarithmic scale to measure the concentration of hydrogen ions (H+) in an aqueous solution.

pH = -log[H+]

The H+ in square brackets denotes that hydrogen ions are the specific chemical species being quantified.

The pH scale measures “potential” differences between solutions. It does so by measuring the negative power of ten (hence a logarithmic scale). In essence, it measures the power of hydrogen in a solution.

What makes 7 neutral?

In any aqueous (water) (aq) solution, there is a given number of hydrogen ions (protons) floating around. The concentration of those protons is what we are measuring.

But why does pH only apply to aqueous solutions? For instance, isn’t soap, which comes in solid form, also a base?

The answer lies in the magic of water (H2O). Water is a polar molecule because the atoms share electrons unequally, and the molecule is asymmetrically shaped. Therefore, H+ (hydrogen ions) are released and accepted by different molecules throughout the fluid. Because of this, pure water acts as both an acid and a base, constantly reacting with itself, creating hydronium (H3O+) and hydroxide (OH-) molecules. Keep in mind that: H+ x H2O = H3O = hydronium ion

This continuous self-reaction is called the dissociation of water. The dissociation of water is a reversible reaction. The molecules always reform into water almost immediately (within a tiny fraction of a second). However, there is always still a small amount of H+ floating around at all times. The exact number of those is an equilibrium constant. In this specific case, the equilibrium constant is called the water dissociation constant, denoted as Kw.

Now, equilibrium constants are formatted in a very specific way, with the concentrations of products over the concentrations of reactants. For example: 2H2O(/) ← → H3O+(aq) + OH-(aq)

The formula for Kw is: Kw = (H3O+)(OH-) over (H2O)2

In other words, Kw = the hydronium concentration times the hydroxide concentration, divided by the water concentration squared.

Because the ions do not contribute significantly to the total mass (contribute only a very small proportion), the water is functionally pure. Pure substances, however, do not have concentrations. So, there would normally be no equilibrium calculation. The formula, therefore, simplifies to: Kw = (H3O+)(OH-)

In other words, Kw = the hydronium concentration multiplied by the hydroxide concentration.

The balanced equation for the dissociation of water shows that hydronium and hydroxide are formed at a 1:1 ratio. That is, their concentrations are equal.

Therefore: Kw = (H3O+)(OH-) = (x)(x)

Therefore: 1.0 x 10-14 = (H3O+)(OH-) = (x)(x)

Which simplifies to: 1.0 x 10-14 = x2

Therefore: (H3O+) = (OH-) = the square root of 1.0 x 10-14

Which means that both concentrations = 1.0 x 10-7 mol/liter at equilibrium.

And, so: pH = -log(1.0 x 10-7) = 7

The pH of pure water is, therefore, 7, and why 7 is neutral!


The pH Logarithm

pH of 14 = 1/10,000,000x less acidic than neutral
pH of 13 = 1/1,000,000x less acidic than neutral
pH of 12 = 1/100,000x less acidic than neutral
pH of 11 = 1/10,000x less acidic than neutral
pH of 10 = 1/1,000x less acidic than neutral
pH of 9 = 1/100x less acidic than neutral
pH of 8 = 1/10x less acidic than neutral
pH of 7 = 1, neutral
pH of 6 = 10x more acidic than neutral
pH of 5 = 100x more acidic than neutral
pH of 4 = 1,000x more acidic than neutral
pH of 3 = 10,000x more acidic than neutral
pH of 2 = 100,000x more acidic than neutral
pH of 1 = 1,000,000x more acidic than neutral
pH of 0 = 10,000,000x more acidic than neutral

Infographic © 2024, Quality Science Labs, LLC. Graphics, illustration, and writing by R. L. Nevinczenko. 

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