When 0.1 M HNO₃ is added to water it dissociates as follow,
HNO₃ → H₃O⁺ + NO₃⁻
Let us calculate the pH of this acid in order to know its strength,
As,
pH = -log [H₃O⁺]
Putting H⁺ concentration value,
pH = -log [0.1]
pH = 1
It is highly acidic,
And being strong acid it completely ionizes in water producing only H₃O⁺ and NO₃⁻.
So,
The concentration of H₃O⁺ = 0.1 M
and,
The concentration of NO₃⁻ = 0.1 M