A solution of ethanoic acid is made by dissolving 3g of pure liquid propanoic acid in 500cm^3 water. Given the pH of the solution is 2.98, calculate Ka.
Before using numbers from the question: 1)What is pH? pH = -log[H+] Square brackets mean concentration of H+ so we need to convert given pH to [H+] in the weak acid coefficient 2) To find the concentration we use the formula triangle to get concentration of acid = moles / volume 3) To find moles we use the mass of pure acid used SOLVE: moles of acid = 3/(36+6=32) = 0.0405 moles concentration = 0.0405/0.5 = 0.81 moles dm -3 Ka = [H+]2 / [HA] = (10-2.98)2 / 0.081 = 1.349 x 10-5
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