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Unformatted text preview: ffering range. pH – pKa = 0.51, it is in the rank
pH - pKa = ± 1 27 All of the following statements concerning
buffers are true EXCEPT
a) buffers resist pH changes upon addition of
small quantities of strong acids or bases.
b) buffers are used as indicators in acid-base
c) the pH of a buffer is close to the pKa of the
weak acid from which it is made.
d) buffers contain appreciable quantities of a
weak acid and its conjugate base.
e) buffers are resistant to changes in pH when
diluted with water. Answer: b Preparing a Buffer
What mass of NH4Cl should be added to 2.55 L of a
0.155 M NH3 to make a buffer having a pH of 9.55?
Kb of NH3 = 1.8× 10-5 pKb = -log 1.8× 10−5 = 4.74
Plan: Kb, pOH and [NH3] ⇒[NH4Cl] ⇒mass NH4Cl
pOH =14 –pH =4.45 [OH-]=10−4.45 =3.55x10−5M
NH3 +H2O ⇄ NH4+ +OH− The fundamental eq. 0.155
? 0.0000355 M
Kb=[NH4+][OH−]/[NH3] [NH4+] can be calculated
[NH4+]= Kb [NH3]/[OH−]
[NH +] = 1.8× 10−5× 0.155 / 3.55× 10−5 = 0.0786 M Preparing a
What mass of NH Cl should be added to
4 2.55 L of 0.155 M NH3 to make a buffer having a pH of 9.55?
Kb = 1.8 × 10-5 Plan: Ka, pH and [NH3] ⇒ [NH4Cl] mass NH4Cl
moles of NH4Cl = moles of NH4+ =
= 0.0786 mol/L x 2.55 L = 0.201 mol
Molar mass of NH4Cl = 53.59 g/mol
mass NH4Cl = 0.201 mol x 53.49 g/mol= 10.6 g
You cannot add isolated NH4+ ions, but a salt such
as NH4Cl (this example) or NH4NO3. These salts are
These Acid-Base Reactions
• Strong acid + strong base
HCl + NaOH → NaCl +
• Strong acid + weak base
HCl + NH3 → NH4Cl
• Weak acid + strong base
HOAc + NaOH → NaOAc +
• Weak acid + weak base
HOAc + NH3 → NH4OAc
HOAc What is the relative
pH before, during,
& after an acid-base
reaction? H3CCOOH = HC2H3O2 = HOAc Titrations & titration curves
Titrations & titration curves pH
pH SA-SB, pH = 7
WA-SB, pH > 7
SA-WB, pH &...
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