Relating pK_a and pK_b Learning Goal: To understand the relation between the str
ID: 529077 • Letter: R
Question
Relating pK_a and pK_b Learning Goal: To understand the relation between the strength of an acid or a base and its pK_a, and pK_b values. The degree to which a weak acid dissociates in solution is given by its acid-ionization constant, K_a. For the generic weak acid, HA, HA (aq) rlhar A^-(aq) + H^+ (aq) and the acid-ionization constant is given by K_a = [A^-][H^+]/[HA] Similarly, the degree to which a weak base reacts with H_2O in solution is given by its base-ionization constant. K_b. For the generic weak base, B, B(aq) + H2O(1) rlhar BH^+(aq) + OH^-(aq) and the base-ionization constant is given by K_b = [BH^+][OH^-]/[B] Another way to express acid strength is by using pK_a: pK_a = -log K_a Another way to express base strength is by using pK_b: pK_b = - log K_b A new potential heart medicine, code-named X-281, is being tested by a pharmaceutical company, Pharma-pill. As a research technician at Pharma-pill, you are told that X-281 is a monoprotic weak acid, but because of security concerns, the actual chemical formula must remain top secret. The company is interested in the drug's K_a value because only the dissociated form of the chemical is active in preventing cholesterol buildup in arteries. To find the pK_a of X-281, you prepare a 0.092 M test solution of X-281 at 25.0 degree C. The pH of the solution is determined to be 3.00. What is the pK_a of X-281? Express your answer numerically. This question will be shown after you complete previous question(s).Explanation / Answer
we know that PH=1/2[PKa-logC]
where C= concentration of acid in normality
given that X-281 is a mono protic weak acid so Molarity= Normality=0.092
substitute the all the values in above equation 3=1/2[PKa-log(0.092)]
6=PKa-(-1.036)
PKa=4.963