Chapter 16.7 and 16.8 Acid –Base Titrations Buffered Solutions

16.7 Acid – Base Titrations Strong acids contain H + Strong acids contain H + Strong bases contain OH - Strong bases contain OH - H + + OH -  H 2 O H + + OH -  H 2 O Neutralization reaction = equal amounts of H+ and OH- results in a neutral solution (pH=7). Neutralization reaction = equal amounts of H+ and OH- results in a neutral solution (pH=7). Titration = involves the delivery of a measured volume of a solution of known concentration (titrant) into the solution being analyzed (analyte) Titration = involves the delivery of a measured volume of a solution of known concentration (titrant) into the solution being analyzed (analyte)

Titrations Standard solution = solution of known concentration Standard solution = solution of known concentration Buret = cylindrical device; allows for a measured amount of liquid to be dispensed. Buret = cylindrical device; allows for a measured amount of liquid to be dispensed. Stochiometry point or equivalence point =Keep adding titrant until all of analyte reacts with titrant, when pH = 7 Stochiometry point or equivalence point =Keep adding titrant until all of analyte reacts with titrant, when pH = 7

Titration Curve or pH curve A titration curve is a plot of the pH of a solution (acid or base) against the volume of titrant added (base or acid). A titration curve is a plot of the pH of a solution (acid or base) against the volume of titrant added (base or acid).

Curve for the titration of a strong acid by a strong base. Figure 17.11

Titration problems What volume of.1 M NaOH is needed to titrate (neutralize) 50.0 mL of.2 M HNO 3 ? What volume of.1 M NaOH is needed to titrate (neutralize) 50.0 mL of.2 M HNO 3 ? We need moles of OH - added to equal moles of H + present. We need moles of OH - added to equal moles of H + present. M = m / L M = m / L M a x V a = m a and M b x V b = m b M a x V a = m a and M b x V b = m b So, M a x V a (m b /m a ) = M b x V b So, M a x V a (m b /m a ) = M b x V b m b /m a (mole to mole ratio) m b /m a (mole to mole ratio)

What volume of.1 M NaOH is needed to titrate (neutralize) 50.0 mL of.2 M HNO 3 ? What volume of.1 M NaOH is needed to titrate (neutralize) 50.0 mL of.2 M HNO 3 ? NaOH + HNO 3  NaNO 3 + HOH NaOH + HNO 3  NaNO 3 + HOH M a V a (b/a) = M b V b M a V a (b/a) = M b V b (.2 M)(50. mL)(1/1) = (.1 M)(V) (.2 M)(50. mL)(1/1) = (.1 M)(V) V=100 mL of NaOH V=100 mL of NaOH

Problem It takes 25mL of H 2 SO 4 to titrtate 30mL of 3 M NaOH. Calculate the molarity of H 2 SO 4. It takes 25mL of H 2 SO 4 to titrtate 30mL of 3 M NaOH. Calculate the molarity of H 2 SO 4. H 2 SO NaOH  Na 2 SO HOH H 2 SO NaOH  Na 2 SO HOH M a V a (b/a) = M b V b M a V a (b/a) = M b V b M a (.026L) (2/1) = (3)(.030L) M a (.026L) (2/1) = (3)(.030L) M a = 1.7 M a = 1.7

Problem When 14.3 mL of M Ca(OH) 2 are neutralized with M HNO 2 what volume of acid would be used? When 14.3 mL of M Ca(OH) 2 are neutralized with M HNO 2 what volume of acid would be used? Ca(OH) HNO 2  Ca(NO 3 ) 2 + 2HOH Ca(OH) HNO 2  Ca(NO 3 ) 2 + 2HOH M a V a (b/a) = M b V b M a V a (b/a) = M b V b (.426)(V) (1/2) = (.573)(14.3 mL) (.426)(V) (1/2) = (.573)(14.3 mL) V acid = mL V acid = mL

Buffers Blood is a buffer. Blood’s pH ranges between 7.35 – If the pH of blood drops below 6.8 or is higher than 7.8, you can not survive. CO 2 + H 2 O  H 2 CO 3  H + + HCO 3 -

16.8 Buffers A buffer is a solution characterized by the ability to resist large changes in pH when limited amounts of acid or base are added to it. A buffer is a solution characterized by the ability to resist large changes in pH when limited amounts of acid or base are added to it. Buffers contain either a weak acid and its conjugate base or a weak base and its conjugate acid. Thus, a buffer contains both an acid species and a base species in equilibrium.

Buffers Buffering capacity = the amount of H+ or OH- that a buffer system can absorb with out significant changes in pH. Buffering capacity = the amount of H+ or OH- that a buffer system can absorb with out significant changes in pH. Aspirin is an acid. When someone overdoses, the body can not neutralize the aspirin. Aspirin is an acid. When someone overdoses, the body can not neutralize the aspirin.

Buffers Weak acid Weak base salt H 2 CO 3 HCO 3 - NaHCO 3 HC 2 H 3 O 2 C2H3O2-C2H3O2-C2H3O2-C2H3O2- NaC 2 H 3 O 2 HCN CN - KCN

Buffers If you add an acid: If you add an acid: H + + HCO 3 -  H 2 CO 3 (pop) (blood) If you add a base: OH - + H 2 CO 3  H 2 O + HCO 3 - OH - + H 2 CO 3  H 2 O + HCO 3 -

Buffers Blood buffers: H 2 CO 3 /HCO 3 - and H 2 PO 4 - /HPO 4 -2 Blood buffers: H 2 CO 3 /HCO 3 - and H 2 PO 4 - /HPO 4 -2 Kidneys: Excrete acidic/basic solutions and back up the neutralization system Kidneys: Excrete acidic/basic solutions and back up the neutralization system Breathing rate: controls amount of CO2 in body. CO 2 + H 2 O  H 2 CO 3 (carbonic acid) Breathing rate: controls amount of CO2 in body. CO 2 + H 2 O  H 2 CO 3 (carbonic acid) Acidosis or alkalosis – can cause death. Acidosis or alkalosis – can cause death.