The pH scale is a logarithmic scale used to measure the acidity or basicity of a solution. The pH of a solution is defined as the negative logarithm of the hydrogen ion concentration ([H+]) in moles per liter (mol/L).

The formula for calculating pH is:

```

pH = -log[H+]

```

When the pH of a solution drops from 7 to 5, the hydrogen ion concentration increases by a factor of 10. This means that the concentration of H+ increases from 1 × 10-7 mol/L to 1 × 10-5 mol/L.

Therefore, the concentration of H+ increases by a factor of 10, or 10 times, when the pH drops from 7 to 5.