Question

Which of the following has the greatest concentration of hydrogen ions? How is this determined?

Human blood at pH 7
Baking soda at pH 9
Black coffee at pH 5
Household ammonia at pH 11
Cola at pH 3

Answer 1

Cola (pH 3)

Explanation:

pH is a shorthand numerical value that represents the concentration of hydrogen ions in solution.

The scale 0-14 represents the usual range of possible concentrations of protons (`H^+`) in an aqueous system.

We can consider `H^+` to mean the same as `H_3O^+` (a water molecule carries
the `H^+`).

`"pH"` is the negative log of hydrogen ion concentration, meaning that:

`"pH" = -log[H_3O^+] = -log[H^+]`

and therefore:

`10^-"pH" = [H^+]`
`10^(-3) = 1.0 × 10^(-3)color(white)(l) "mol/L"`

Let's simplify:

`"pH"color(white)(m) [H^+] ("mol/L")`
`color(white)(ll)3color(white)(mml)1 × 10^(-3)`
`color(white)(ll)5color(white)(mml)1 × 10^(-5)`
`color(white)(ll)7color(white)(mml)1 × 10^(-7)`
`color(white)(ll)9color(white)(mml)1 × 10^(-9)`
`color(white)(l)11color(white)(mm)1 × 10^(-11)`

This is counter-intuitive without the background information (i.e. the negative log) but, as you increase pH, you decrease the concentration of `H^+`.

As you decrease the pH, you increase the concentration of `H^+`.