A few drop of conc HCL are added to about 10cm$^3$ of a solution of PH 3.4. The PH of the resulting mixture is?

Analysis

The question asks how the pH of a solution changes when a strong acid (concentrated hydrochloric acid, HCl) is added to it.

Concept Explanation

• pH Scale: The pH scale measures the acidity or alkalinity of a solution. It ranges from 0 to 14. • pH < 7 is acidic. • pH = 7 is neutral. • pH > 7 is alkaline (basic).
• pH Definition: pH is defined as $-\log_{10}[H^+]$, where $[H^+]$ is the concentration of hydrogen ions.
• Relationship: As the concentration of hydrogen ions ($H^+$) increases, the pH value decreases.
• Effect of HCl: Concentrated HCl is a strong acid that dissociates completely in water to release a high concentration of $H^+$ ions. Adding this to any solution will significantly increase the total concentration of $H^+$ ions in that mixture.

Step-by-Step Solution

1. Initial State: The solution has a pH of 3.4, meaning it is already acidic.
2. Action: You are adding concentrated HCl, which is a source of many $H^+$ ions.
3. Reaction: The addition of $H^+$ ions increases the acidity of the solution.
4. Result: Since the concentration of $H^+$ ions has increased, the pH value must shift further away from 7 towards 0. Therefore, the new pH will be a value lower than 3.4 (e.g., it might drop to 2.0 or 1.5 depending on the amount added).

Conclusion

Because the addition of a strong acid increases the hydrogen ion concentration, the pH value will decrease. Comparing this to the options provided, the resulting pH must be less than 3.4.

Exam Tip: Always remember the "pH Seesaw": If the acidity ($H^+$ concentration) goes up, the pH number goes down. If the alkalinity ($OH^-$ concentration) goes up, the pH number goes up.