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A comprehensive guide on calculating ph values for monoprotic acids and bases, including strong acids and bases, weak acids and bases, and acid-base conjugates. It covers the conditions for neutral systems, strong acid and base solutions, and weak acid and base solutions. The document also explains the concepts of degree of dissociation and buffer capacity.
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Acid/base system
Examples Calculation of pH / pOH Conditions
Pure water and neutral salts
H 2 O, KCl, NaNO3, CsClO 4 ...
[H+] = [OH–] = K (^) W under any conditions
Strong acids HCl, HNO 3 , [H+] = cstrong acid cstrong acid > 10–6^ M H 2 SO 4 (only the first deprotonation step), HClO 4 , CF 3 SO 3 H [H ]
[H +^ ]= cstrong acid+ W+
K under any conditions
Strong bases NaOH, KOH, [OH–] = cstrong base cstrong base > 10–6^ M RbOH, CsOH...
[OH ]
[OH −^ ]= cstrong base+ W−
K under any conditions
c [H ] [H ] a weakacid+
Monoprotic weak acids and acidic salts
NH 4 Cl (NH 4 +)... any monoprotic acid with a K a
[H+] = K (^) a ⋅cweakacid cweak acid^ >>>^ K a
c [OH ] [OH ] b weakbase −
− − (^) = K ⋅ − under any conditions
Monoprotic weak bases and basic salts
CH 3 COONa (CH 3 COO–)... any monoprotic base with a K b
[OH–] = K (^) b ⋅cweakbase cweak base^ >>>^ K b
[H+] = cstrong acid cstrong acid >> cweak acid
Strong and weak acid in the same solution [H ]
c [H ] [H ] cstrong acid a weakacid+
[OH–] = cstrong base cstrong base >> cweak base
Strong and weak base in the same solution [OH]
c [OH ] [OH ] cstrong base b weakbase −
− − (^) = + K ⋅ − under any conditions
Acidic buffers
CH 3 COONa together Basic buffers
NH 3 + NH 4 Cl together
[H+] = K a weak base
weakacid c
c ⋅ = K a weak base
weakacid n
n ⋅ ...
pH = p K a + log conjugateacid/weak acid
weakbase/conjugatebase c
c
General equations for pH calculations:
pH = –log[H+] pOH = –log[OH–] [H+] = 10–pH^ [OH–] = 10–pOH
[H+][OH–] = K W in any aqueous solution pH + pOH = p K W = –log K W
K a K b = K W for any conjugate acid-base pair p K a + p K b = p K W
Some other concepts:
The concept of degree of dissociation (α) is used when only a weak acid or base are present in the solution, no other chemicals affecting the pH.
Degree of dissociation = totalnumberofmoles
numberofmolesofthedissoc. form
Buffer capacity (unit: mol, mol/dm^3 , dm^3 or cm^3 ) is the amount of H+^ and OH–^ ions (i.e. the amount of strong acid and strong base) which should be added to 1 dm^3 of buffer solution to change its pH by 1 unit.
Calculation of buffer capacity for strong acid (nstrong acid):
[H+]buffer – 1 = K a weakbase strong acid
weakacid strongacid n n
n n −
Calculation of buffer capacity for strong base (nstrong base):
[H+]buffer + 1 = K a weakbase strong base
weakacid strongbase n n
n n