A 0.100 M HCl solution is prepared. What are [H+] and pH?

A 0.100 M HCl solution is prepared. What are [H+] and pH?

• A. [H+] = 0.100 M; pH = 2.00
• B. [H+] = 0.0010 M; pH = 3.00
• C. [H+] = 0.100 M; pH = 1.00
• D. [H+] = 0.0100 M; pH = 2.00

Answer: (C)

A strong acid like HCl dissociates completely in water, so every HCl molecule yields one H+ ion. That means the hydrogen-ion concentration [H+] directly equals the acid concentration, 0.100 M. The pH is defined as pH = −log[H+], so pH = −log(0.100) = 1.00. This is why the correct result is [H+] = 0.100 M and pH = 1.00. If the concentration were 0.0100 M, the pH would be 2.00, and if it were 0.0010 M, the pH would be 3.00—only the stronger acid with the given concentration yields pH 1.00.

A strong acid like HCl dissociates completely in water, so every HCl molecule yields one H+ ion. That means the hydrogen-ion concentration [H+] directly equals the acid concentration, 0.100 M. The pH is defined as pH = −log[H+], so pH = −log(0.100) = 1.00. This is why the correct result is [H+] = 0.100 M and pH = 1.00. If the concentration were 0.0100 M, the pH would be 2.00, and if it were 0.0010 M, the pH would be 3.00—only the stronger acid with the given concentration yields pH 1.00.