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pH of a solution of a strong acid is 5.0. What will be the pH of the solution obtained after diluting the given solution 100 times?

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As per the information given in the question, we know that the pH=5.0

pH = 5 means [H+]=10−5 M, 

On diluting it 100 times, we get

[H^{+}]=\frac{10^{-5}}{100}=10^{-7}mol/L

This should give pH = 7 but it cannot be so because solution is acidic and pH should be less than 7

 The reason is that [H+] from H2O cannot be neglected. Thus, total [H+]=10−7M (from HCl) + 10−7M (from H2O ) =2×10−7M

 

∴pH=−log(2×10−7)= 7−0.3010=6.699

 

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