Concept:pH is the negative logarithm of the hydronium ion concentration, so it directly gives [H3O+] when reversed.Explanation:The relationship between pH and [H3O+] is: pH=−log[H3O+] Rearranging the equation gives: [H3O+]=10−pH Substitute the given pH=4.25: [H3O+]=10−4.25 Express the exponent as a sum: 10−4.25=10−4×10−0.25 The value 10−0.25 is approximately 0.5623, so: [H3O+]=10−4×0.5623[H3O+]=5.623×10−5MAnswer:The correct option is B: 5.623×10−5M.