# 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

Aug 30, 2016

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 ${H}_{3} {O}^{+}$ (a water molecule carries
the ${H}^{+}$).

$\text{pH}$ is the negative log of hydrogen ion concentration, meaning that:

$\text{pH} = - \log \left[{H}_{3} {O}^{+}\right] = - \log \left[{H}^{+}\right]$

and therefore:

${10}^{-} \text{pH} = \left[{H}^{+}\right]$
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 ${H}^{+}$.