Question

aspirin, a commonly used pain reliever, is a weak organic acid whose molecular formula may be written as HC9H7O4. an aqueous solution of aspirin has a total volume of 350ml and contains 1.26g of aspirin. the pH of the solution is found to be 2.60. what does Ka stand for, what do you need to calculate Ka

Answer 1

Molar mass of aspirin,

MM = 9*MM(C) + 8*MM(H) + 4*MM(O)

= 9*12.01 + 8*1.008 + 4*16.0

= 180.154 g/mol

mass(C9H8O4)= 1.26 g

number of mol of C9H8O4,

n = mass of C9H8O4/molar mass of C9H8O4

=(1.26 g)/(180.154 g/mol)

= 0.006994 mol

volume , V = 350 mL

= 0.35 L

Molarity,

M = number of mol / volume in L

= 0.006994/0.35

= 0.020 M

This is the concentration of aspirin

use:

pH = -log [H3O+]

2.6 = -log [H3O+]

[H3O+] = 2.512*10^-3 M

HC9H7O4 dissociates as:

HC9H7O4 -----> H+ + C9H7O4-

2*10^-2 0 0

2*10^-2-x x x

Ka = [H+][C9H7O4-]/[HC9H7O4]

Ka = x*x/(c-x)

Ka = 2.512*10^-3*2.512*10^-3/(0.02-2.512*10^-3)

Ka = 3.608*10^-4

Ka is called acid dissociation constant

Answer : 3.608*10^-4