**Answer:**

**6.37.**

**Explanation:**

What is given?

[H (+)] = 4.25 x 10⁻⁷ mol/dm³

Step-by-step solution:

First, let's see the formula to calculate the pH based on the hydrogen ion concentration (H (+)):

*Logarithm must be in base 10!

Our given concentration of hydrogen ion is 4.25 x 10⁻⁷ mol/dm³. Remember that the units of concentration must be in M (mol/L) but in this case, 1 dm³ equals 1 L, so based on this, we have that:

So, we can replace this value in the formula of pH:

The pH of milk would be **6.37** which means that is slightly acid (weak acid).