The pH of a solution of hydrochloric acid is 4. The molarity of the solution is

The correct answer is Option 2 (0.0001 M).

Key Points

• The pH of a solution is defined as the negative logarithm of the hydrogen ion concentration: pH = -log[H⁺].
• A solution with a pH of 4 implies that the hydrogen ion concentration, [H⁺], is 10^-4 M.
• Since hydrochloric acid (HCl) is a strong acid, it completely dissociates in water, meaning that the molarity of HCl is equal to the hydrogen ion concentration.
• Therefore, the molarity of the HCl solution is 0.0001 M, which corresponds to Option 2.
• This calculation assumes ideal conditions and no significant dilution or external interference.

Additional Information

• pH Scale:
  • The pH scale ranges from 0 to 14, where 7 is neutral.
  • pH values below 7 indicate acidity, while values above 7 indicate basicity (alkalinity).
  • The pH scale is logarithmic; each unit change represents a tenfold change in [H⁺].
• Strong Acids:
  • Strong acids, like HCl, HNO₃, and H₂SO₄, completely ionize in water.
  • For strong acids, the molarity of the acid is equal to the molarity of [H⁺].
• Logarithmic Calculations:
  • For a hydrogen ion concentration of 10^-x, the pH is simply x (e.g., [H⁺] = 10^-4 implies pH = 4).
  • Logarithmic properties are essential for understanding pH calculations.
• Applications of pH:
  • pH is crucial in fields like chemistry, biology, agriculture, and medicine.
  • It helps monitor acidity in soil, water quality, and human blood (normal blood pH is ~7.4).