The Danish chemist Sørensen introduced the concept of pH in 1909 to measure the acidity of solutions while working in the Carlsberg brewery in Copenhagen - Leaving Cert Chemistry - Question b - 2017

pH is defined as the negative logarithm of the concentration of hydrogen ions in a solution. Mathematically, it is expressed as:

$pH = - ext{log} [H^+]$

The relationship between pH and pOH is given by the equation:

$pH + pOH = 14$

This means that if you know one of the values, you can easily calculate the other.

To calculate the pH of a 0.3 M HCl solution, which is a strong acid that completely dissociates in solution:

First, determine the hydrogen ion concentration:
$[H^+] = 0.3 ext{ M}$

Next, calculate pH using the formula:

\ pH = 0.523$$

The solutions in order of their increasing acidity are:

1. Mouthwash (pH 8.0)
2. Tap-water (pH 7.5)
3. Coffee (pH 5.0)
4. Beer (pH 3.9)