Procedure for calculating pH of a titration Volume of titrant needed is based on amount of analyte.
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Procedure for calculating pH of a titration Volume of titrant needed is based on amount of analyte
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Transcript of Procedure for calculating pH of a titration Volume of titrant needed is based on amount of analyte.
Procedure for calculating pH of a titration
Volume of titrant needed is based on amount of analyte
Exactly 100 mL of 0.10 M HNO2 are titrated with a 0.10 M NaOH solution. What is the pH at the equivalence point ?
HNO2 (aq) + OH- (aq) NO2- (aq) + H2O (l)
start (moles)
end (moles)
0.01 0.01
0.0 0.0 0.01
NO2- (aq) + H2O (l) OH- (aq) + HNO2 (aq)
Initial (M)
Change (M)
Equilibrium (M)
0.05 0.00
-x +x
0.05 - x
0.00
+x
x x
[NO2-] =
0.010.200 = 0.05 MFinal volume = 200 mL
Kb =[OH-][HNO2]
[NO2-]
=x2
0.05-x= 2.2 x 10-11
0.05 – x 0.05 x 1.05 x 10-6 = [OH-]
pOH = 5.98
pH = 14 – pOH = 8.02
A sample of 0.1276 g of an unknown monoprotic acid was dissolved in 25.0 ml of water and titrated with 0.0633 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 = (0.0633 moles/L)(0.0184 L) = 0.00116 moles
Moles Base = Moles Acid @ Equiv. Point
MW Acid = 0.1276 g / 0.00116 moles = 110. g/mole
After 10.0 ml NaOH solution was added, the pH was 5.87. What is the Ka of the unknown acid?
pKa = pH - log [A-][HA]
10.0 ml NaOH => (0.0633 moles/L)(0.0100 L) = 0.000633 moles = moles A-
Moles HA = (total moles HA) - (moles A- )
= 0.00116 moles - 0.000633 moles
= 0.000527 moles
pKa = pH - log [A-][HA]
= 5.87 - log
[A-][HA]
= moles / Vmoles / V
Volumes are the same since they are in the same
solution
0.000633
0.000527= 5.87 - 0.079
pKa = 5.79
Ka = 10-5.79 = 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 HNO2 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 pKa acids (stronger weak acids) change color
@ low pH
High pKa acids (weaker weak acids) change color
@ high pH
Use small amount of indicator so experiment is
not altered.
