Calculate the pH after 0.015 mole of gaseous HCl is added to 250.0 mL of a buffe

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Explanation / Answer

For these types, which are buffered solutions, of problems you need to use the henderson-Hasselbalch equation: pH = pKa + log([base]/[acid]) The reaction here is: NH3 + HCl ---> NH4(+) + Cl- and the base is NH3 and the acid for use in the Herderson equation is NH4+. The pKa of NH4+ is 9.25 (from CRC) The next thing that needs to be determined is the number of mol of each species in the reaction. mol of NH3 (init) = 0.75 mol/l * 0.250 l = 3 mol mol of NH4+ (init) = 0.15 mol/l * 0.2822 l = 0.60 mol Now adding 0.015 mol of HCl will consume 0.015 mol of NH3 and produce 0.015 mol of NH4+ so after the reaction takes place you will have: mol of NH3 (final) = 3 - 0.015 = 2.99 mol mol of NH4+ (final =0.60 + 0.015 = 0.62 mol Now determine the concentrations: [NH3] = 2.99/0.250 = 11.94 mol/l [NH4+] = 0.62/0.250 = 2.46 mol/l now use the Henderson-Hasselbalch equation to determine the pH of the final solution. pH = 9.25 + log(11.94/2.46) pH = 9.94

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