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pH or pOH is the measure of acidity or alkalinity of a solution.
pH is calculated by taking negative logarithm of hydronium ion concentration.
Ph=-logH3O+
Putting in the values:
8.43=-log[H3O+]
[H3O+]=10^-8.43=3.7*10^-9
Thus H3O+ concentration is 10^-8.43