Buffering against pH changes in biologicalsystems. A compound has a pKa of 7.4.

Question

Buffering against pH changes in biologicalsystems. A compound has a pKa of 7.4. To 100 mL of a 1.0 Msolution of this compound at pH 8.0 is added 30 mL of 1.0 Mhydrochloric acid. The resulting solution is pH: (a.) 6.5 (b.) 6.8 (c.) 7.2 (d.) 7.4 (e.) 7.5 (In order to receive credit, all calculations that are used tolead to the answer must be shown. It should be obvious whatassumptions and mathematical relationships were used to arrive atthe answer.) A compound has a pKa of 7.4. To 100 mL of a 1.0 Msolution of this compound at pH 8.0 is added 30 mL of 1.0 Mhydrochloric acid. The resulting solution is pH: (a.) 6.5 (b.) 6.8 (c.) 7.2 (d.) 7.4 (e.) 7.5 (In order to receive credit, all calculations that are used tolead to the answer must be shown. It should be obvious whatassumptions and mathematical relationships were used to arrive atthe answer.)

Explanation / Answer

Formula :                pH = pKa + log ( No.of mols of salt / No.of mols of acid) No.of mols of salt = 0.1 L * 1 M                              =0.1 mols No.of mols of acid = 0.1 L * 1M                              =0.1 mol When a strong acid is added to the acidic buffer , no.of molsof salt decreases and no.of mols of acid increases. No.of mols of HCl = 0.03 L * 1.0 M                               =0.03 mols After adding 0.03 mols of HCl : No.of mols of Salt = 0.1 mol - 0.03 mol                              = 0.07 mols No.of mols of acid = 0.1 mol +0.03 mol                               =0.13 mol pH     = 7.4 + log ( 0.07 /0.13)              = 7.18              = 7.2 pH is 7.2 .

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