Question

what is the formula to calculate ph?
a) $\ce{ph = logh_2o}$
b) $\ce{ph = -logh_2o}$
c) $\ce{ph = -logh_3o^+}$
d) $\ce{ph = logh_3o^+}$

Explanation:

pH is defined as the negative base-10 logarithm of the hydronium ion ($\text{H}_3\text{O}^+$) concentration in a solution. Options a and b use water ($\text{H}_2\text{O}$) concentration, which is irrelevant to pH. Option d uses a positive logarithm, which would produce negative values for typical hydronium concentrations, contradicting the pH scale.

Answer:

c) $\text{pH} = -\log[\text{H}_3\text{O}^+]$