Hello! Such questions are answered by using concept from chemical equilibrium and pH formulas. First, you are required to find the concentration of H+ or H3O+ ions at concentration by using chemical equilibrium secondly you will calculate pH for particular H+ concentration by using -log [H+] formula. Below the equation shows the reaction of butanoic acid with water producing butyrate and hydronium ions. Initially when the reaction is at time = 0 no H3O+ is produced, then, reaction starts and equilibrium is attained at which unknown (x) amount of H3O+ is formed. This unknown concentration is calculated as shown below (i don't know much maths so, i use online calculator). When concetration of H3O+ is in hand then you can simply calculate pH.
Also, before starting the calculation you should find out Molarity of given acid by following method,
Molarity = moles ÷ dm³
Molarity = (mass ÷ M. Mass) ÷ dm³
M. Mass of Butyric acid = 88 g/mol
So, Molarity = (8.7 g ÷ 88 g/mol) ÷ 1 dm³
Molarity = 0.0988 mol/dm³
