Procedure for calculating pH of a titration Volume of titrant needed is based on amount of analyte

Exactly 100 mL of 0.10 M HNO 2 are titrated with a 0.10 M NaOH solution. What is the pH at the equivalence point ? HNO 2 (aq) + OH - (aq) NO 2 - (aq) + H2O (l) start (moles) end (moles) NO 2 - (aq) + H 2 O (l) OH - (aq) + HNO 2 (aq) Initial (M) Change (M) Equilibrium (M) x-x+x+x x x+x xx [NO 2 - ] = = 0.05 M Final volume = 200 mL K b = [OH - ][HNO 2 ] [NO 2 - ] = x2x x = 2.2 x – x  0.05x  1.05 x = [OH - ] pOH = 5.98 pH = 14 – pOH = 8.02

A sample of g of an unknown monoprotic acid was dissolved in 25.0 ml of water and titrated with M NaOH solution. The volume of the NaOH required to get to the equivalence point was 18.4 ml. What is the molar mass of the acid? Moles Base = ( moles/L)( L) = moles Moles Base = Moles Equiv. Point MW Acid = g / moles = 110. g/mole After 10.0 ml NaOH solution was added, the pH was What is the K a of the unknown acid? pK a = pH - log [A - ] [HA] 10.0 ml NaOH => ( moles/L)( L) = moles = moles A -

Moles HA = (total moles HA) - (moles A - ) = moles moles = moles pK a = pH - log [A - ] [HA] = log [A - ] [HA] = moles / V moles / V Volumes are the same since they are in the same solution = pK a = 5.79 K a = = 1.62 x 10 -6

Acid-Base Indicators HIn (aq) H + (aq) + In - (aq)  10 [HIn] [In - ] Color of acid (HIn) predominates  10 [HIn] [In - ] Color of conjugate base (In - ) predominates 16.5

The titration curve of a strong acid with a strong base. 16.5

Which indicator(s) would you use for a titration of of HNO 2 versus KOH ? Weak acid titrated with strong base. At equivalence point, will have conjugate base of weak acid. At equivalence point, pH > 7 Use cresol red or phenolphthalein Low pK a acids (stronger weak acids) change low pH High pK a acids (weaker weak acids) change high pH Use small amount of indicator so experiment is not altered.