(1): 50.0 mL(2): 75.0 mL(3): 25.0 mL(4): 100.0 mLAnswer: (3)NaOH: 0.050 L x 0.100 mol/L = 0.005 mol0.200 mol1 L=0.005 molX LX L=0.005 mol x 1L0.200 mol=0.025 L= 25 mL
Methylamine has a pKbof 3.4. A 50.0 mL solution ofmethylamine is titrated to the stoichiometric point with 65.4 mLof 0.025 Mhydrochloric acid. What is the pH of the resultingsolution?Type of problem:Titration to endpoint (weak base)Determine concentration of final samplepH of final sample =pH of the solution of a salt of a weakbase and a strong acid of known concentrationKnowns:Volume of basepKbof base (conversion to pKa)Volume and concentration of titrantWhat happens?CH3NH2+ HCl [CH3NH3]+Cl–Final volume = 115.4 mL It contains:0.025 mol/L x 65.4 x 10–3L = 1.635 x 10–3mol of [CH3NH3]+Final concentration: 1.635 x 10–3mol/0.1154 L = 0.0142 mol/LRemaining task: Need to calculate the pH of a 0.0142 Msolution of [CH3NH3]+!Methylamine has a pKbof 3.4. A 50.0 mL solution ofmethylamine is titrated to the stoichiometric point with 65.4 mLof 0.025 Mhydrochloric acid. What is the pH of the resultingsolution?
Lecture 23,Knowledge and Skills• pH of fluids in the body• pH of aqueous solutions in the environment• Acid-base properties of carbon dioxide• pH buffer in blood• Chemistry of tooth decaySome Information about Carbonic AcidCO2+ H2O H2CO3[H2CO3][CO2]Kh= = 1.70 ±10²3Hydration equilibriumOnly a small fraction of the hydrated carbon dioxide, i.e.,carbonic acid,dissociates to give hydronium ions and bicarbonate.H2CO3+ H2O H3O++ HCO3–[H3O+][HCO3–][H2CO3]Ka1= = 2.5±10²4Acid dissociation, bicarbonate formationOnly a tiny fraction of the bicarbonate dissociates to give hydronium ionsand carbonate.HCO3–+ H2O H3O++ CO32–[H3O+][CO32–][HCO3–]Ka2= = 4.69±10²11pKa= 3.60pKa= 10.33Bicarbonate dissociation, carbonate formationOnly a small fraction of the dissolved carbon dioxide is hydrated to form carbonic acid.
Some Information about Carbonic AcidCO2+ H2O H2CO3Kh= 1.70 ±10²3It is often practical to quote the dissociation of carbonic acid relative tothe total amount of carbon dioxide dissolved.H2CO3+ H2O H3O++ HCO3–Ka1= 2.5±10²4CO2+ 2 H2O H3O++ HCO3–Ka1(app) = 4.6±10²7 The H3O+ion concentration generated by a certain amount of dissolved CO2. The Ka1(apparent) is often found in tables. Be sure to know what the value means!Ka1(app) is useful in calculating the pH of CO2solutions. However, carbonic acid is NOT such a weak acid! Hydrolysis of bicarbonate to form carbonate (Ka2= 4.69±10²11) does notcontribute significantly to the pH of a carbonic acid solution.pKa1(app) = 6.34Some Information about Carbonic AcidIntrinsic pKa of carbonic acid:3.60Apparent pKa of carbonic acid:6.34HOCOOHHOCCOHHHAcetic acid:4.75HOCHOFormic acid:3.75HOCOBenzoic acid:4.19
Some Information about Carbonic AcidCO2+ H2O H2CO3[H2CO3][CO2]Kh= = 1.70 ±10²3Hydration equilibriumThe rate of equilibration is slow:k(forward)=0.039 L mol²1s²1k(reverse)= 23 s²1Red blood cells contain an enzyme that speeds up this reaction: