What is the pH of a 2.1 M solution of HClO4?

Since HCl04 is a strong acid, being [H+], and a molarity of 2.1 M.

To solve for pH:

pH = - log (M) = - log (2.1M) = - 0.32, which is clearly a negative number.

However, to verify the answer, just use the pH meter in determining the pH of the solution.