What is the hydrogen concentration of a solution with a pH of 2.1?

8*10^ (-3) mol/L

Explanation:

The formula for finding the hydrogen ion concentration is ...

[H⁺] = 10^-ph

[H⁺] = 10^ (-2.1)

[H⁺] = 0.00794 ... mol/L = 8*10^ (-3) mol/L

The answer is to 1 significant digit because the pH has one decimal place.