In a 0.1000M solution of methanoic acid, [H+]=4.2 x 10^-3M. Calculate the Ka of this acid.

In order to determine the Ka of the compound, we can use the expression for the calculation of pH for weak acids which is expressed as:

pH = - 0.5 log Ka

-log (H+) = - 0.5 log Ka

-log (4.2x10^-3) = - 0.5 log Ka

Ka = 1.764x10^-5

Hope this answers the question.