Does the pH value, the negative of the exponent of the hydronium concentration, always equals the molarity of the acid?I do not mean that the molarity would be negative, just the number. Ex: 10^-5M...

Does the pH value, the negative of the exponent of the hydronium concentration, always equals the molarity of the acid?

I do not mean that the molarity would be negative, just the number.

Ex: 10^-5M = pH of 5

Is this always the case? Are there exceptions? Please explain.

Asked on by takenotes

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ncchemist | eNotes Employee

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By definition, the pH of a solution is the negative of the log of the concentration of protons in the solution.  This can be stated mathematically as pH = -log[H+].  You are looking only at the exponent of the concentration in scientific notation to get the pH value.  This will only work if the concentration is the integer 1 raised to an exponential power.  pH is not equal to the molarity of the acid, it is equal to the negative log of the molarity.  So do not just look at the exponent to get the pH value.  For example, if the proton concentration is 6x10^-5, then the pH = -log(6x10^-5) = -(-4.22) = 4.22.  Just looking at the exponent will give you the wrong answer.

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