Dilution is a common technique in chemistry to reduce the concentration of a solution. If you start with 10⁻⁶ M hydrochloric acid (HCl) and dilute it by 100 times, you’re significantly lowering the concentration of hydrogen ions in the solution.
Step-by-Step Explanation
Initial Concentration of HCl
- Starting concentration = 10⁻⁶ M
- HCl is a strong acid, so it dissociates completely in water:
HCl→H⁺+Cl⁻\text{HCl} \rightarrow \text{H⁺} + \text{Cl⁻}HCl→H⁺+Cl⁻
Dilution by 100 Times
Dilution formula:
C2=C1dilution factor=10−6100=10−8 MC_2 = \frac{C_1}{\text{dilution factor}} = \frac{10^{-6}}{100} = 10^{-8} \text{ M}C2=dilution factorC1=10010−6=10−8 M
So, after dilution, the new concentration of H⁺ ions is:
[H⁺]=10−8 M[\text{H⁺}] = 10^{-8} \text{ M}[H⁺]=10−8 M
What Does It Mean?
When the concentration of H⁺ ions becomes 10⁻⁸ M, it’s very close to the concentration of H⁺ in pure water, which is also about 10⁻⁷ M due to water’s autoionization:
H₂O⇌H⁺+OH⁻\text{H₂O} \rightleftharpoons \text{H⁺} + \text{OH⁻}H₂O⇌H⁺+OH⁻
At this level, the contribution from water itself becomes significant and affects the overall pH.
pH Consideration
- pH = –log[H⁺]
- At 10⁻⁸ M, pH = 8, but this isn’t entirely accurate due to water’s autoionization.
- The actual pH will be slightly less than 7, because both water and the acid contribute H⁺ ions.
So, after dilution, the solution becomes nearly neutral, but still slightly acidic.
Final Result Summary
- Initial HCl concentration: 10⁻⁶ M
- After 100× dilution: 10⁻⁸ M
- Approximate pH: Close to 6.9–7.0
- Conclusion: The solution becomes extremely dilute, nearly neutral, but technically still acidic.
💡 Tip: When dealing with very dilute strong acids, always consider water’s autoionization for accurate pH values.