Three drops of a 1.0 mol $dm^{-3}$ solution of HCl was added to 20 cm3 of a solution of pH6.4. The pH of the resulting solution will be

Analysis

The question asks us to determine the effect of adding a strong acid (HCl) to a solution that already has a pH of 6.4.

Concept Explanation

• pH Scale: The pH scale measures the acidity or alkalinity of a solution. A pH of 7 is neutral, values below 7 are acidic, and values above 7 are alkaline.
• pH Definition: pH is defined as $-\log_{10}[H^+]$. As the concentration of hydrogen ions $[H^+]$ increases, the pH value decreases.
• Effect of Adding Acid: HCl is a strong acid that dissociates completely in water to release hydrogen ions ($H^+$). When you add an acid to any aqueous solution, you are increasing the total concentration of $H^+$ ions in that solution.
• Result: Since the concentration of $H^+$ increases, the pH must shift towards a lower value (becoming more acidic).

Step-by-Step Solution

1. Identify the initial state: The solution has a pH of 6.4, which is slightly acidic.
2. Identify the additive: We are adding HCl, which is a source of $H^+$ ions.
3. Determine the impact: Adding $H^+$ ions to a solution increases the acidity.
4. Relate to pH: Because pH is inversely related to the concentration of $H^+$ ions, an increase in $H^+$ concentration results in a decrease in the pH value.
5. Compare with options: • If the pH was 6.4, adding an acid will make the pH value drop below 6.4 (e.g., to 6.0, 5.0, etc., depending on the volume and concentration). • Therefore, the new pH will be less than 6.4.

Conclusion

Adding a strong acid to a solution increases the hydrogen ion concentration, which mathematically lowers the pH value.

Exam Tip

Always remember the "pH Seesaw":

• If you add Acid, the pH goes Down.
• If you add Base/Alkali, the pH goes Up. Think of it as a seesaw where the pH value is on one side and the acidity is on the other!