Concept:pH is the negative logarithm (base 10) of the hydrogen ion concentration. HCl is a strong acid, meaning its molarity is directly equal to the hydrogen ion concentration.
Formula:$$ [\text{H}^+] = 10^{-\text{pH}} $$
Solution:- The given pH is 4.
- Plug this into the inverse logarithm formula: \( [\text{H}^+] = 10^{-4} \text{ M} \).
- Convert scientific notation to decimal form: \( 10^{-4} = 0.0001 \text{ M} \).
- Since HCl fully dissociates (\( \text{HCl} \rightarrow \text{H}^+ + \text{Cl}^- \)), the initial molarity of the HCl solution is exactly 0.0001 M.
Why other options are incorrect:Option D (4) is the pH value itself, not the concentration. Options B and C introduce a '4' into the significant figures, which is a common math error confusing the exponent with the coefficient.
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