Consider a solution containing .2 M HOAc (Ka = 1.8X10^-5) and.10 M NaOAc. Calculate the pH of this solution and calculate the pHof the solution after .020 mol of HCl (g) is dissolved in 1 L ofthis solution. The acid HOCl has a pKa value of 7.50. Calculate the pH of asolution containing .25 M HOCl and .75 M NaOCl. Here are two problems that I need help with dealing with pHvalues. Thanks. Consider a solution containing .2 M HOAc (Ka = 1.8X10^-5) and.10 M NaOAc. Calculate the pH of this solution and calculate the pHof the solution after .020 mol of HCl (g) is dissolved in 1 L ofthis solution. The acid HOCl has a pKa value of 7.50. Calculate the pH of asolution containing .25 M HOCl and .75 M NaOCl. Here are two problems that I need help with dealing with pHvalues. Thanks.
Explanation / Answer
a ) Formula : pH = pKa + log ( No.of mols of salt / No.of mols of acid) Initial mols of salt =0.10 mols Initial mols of acid = 0.20 mols pKa = 4.7447 pH = 4.7447 + log ( 0.10 / 0.20 ) = 4.44 When a strong acid is added to the acidic buffer no.of mols ofsalt decreases and no.of mols of acid increases. No.of mols of salt = 0.10 mols - 0.02 mols =0.08 mols No.of mols of acid = 0.2 mols - 0.02 mols =0.18 mols pH= 4.7447 + log ( 0.08 / 0.18 ) = 4.39 b) Formula : pH = pKa + log [salt]/[acid] [salt] = 0.75 M [acid] = 0.25 M pKa = 7.50 pH = 7.50 + log ( 0.75 / 0.25 ) = 7.02