Question

How many grams of HCOOH are needed to make 2.0 liters of aqueous solution of pH=3.8?Be sure to back correct from equilibrium molecular acid molarity to starting molecular acid molarity.

Answer 1

Let Molarity of HCHO2= C formic acid is a weak acid, it dissociate partially into acetate and H+ in aqueous media as shown below HCHO2 (aq) <-----> CHO2- (aq) + H+ (aq) Initial Change +x Equilibrium 0 C-X Given Ka= 1.80E-04 pH= 3.80 Weak acid dissociation constant Ka= ( CHO2-)(H+)/(HCHO2) given pH = 3.80 1.80E-04 = x * X (C-x) but pH = -log [H+] (C-x) = x * x 1.80E-04 log [H+]=-pH X + x*x = 1.80E-04 | [H+] = 10^-pH C= 0.00016 + 0.00016 * 0.00016 = 1.80E-04 = 10^(-3.80 c = 0.00030 M 0.00016 from ICE table x= H+ = 0.00016 Concentration of HOOH = 0.00030 M Now mass of HCOOH = Molarity * Volume * Molar Mass 0.00030 * 2.0 * 46.03 0.0274