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22 Cards in this Set
- Front
- Back
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Auto Dissociation of Water
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Water is amphoteric
H₂O + H₂O ↔ H₃O⁺ + OH⁻ Kw = [H₃O⁺][OH⁻] = 1x10⁻¹⁴ @ standard conditions (25° C and 1 atm) pKw = pH + pOH = 14 @standard conditions (25° C and 1 atm) describes range of pH and pOH scales |
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pH/pOH scales
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pH - most acidic is 0 least acidic 14
pOH - most basic is 0 least basic is 14 neutral solutions have pH and pOH of 7 (equal quantities of both) |
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pKa/pKb conjugates
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Kw = (Ka)(Kb) = 1x10⁻¹⁴
@standard conditions pKa + pKb = 14 compares strength of conjugate acids and bases Can ONLY be used to compare conjugates |
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Types of pH/pOH problems
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Strong Acid - SA
Strong Base -SB Weak Acid - WA Weak Base - WB mixture of acid and base - Neutralization of SA with SB or vice versa - Neutralization of SA with SB or vice versa - Buffered solutions (mix of conjugate acids and bases) |
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Strong Acid pH calculation
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pH = -log [SA]
watch out for H₂SO₄ as it is diprotic and you will have to multiply [SA] by 2 - can get negative pH |
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estimating -log [x]
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if x = 1 x 10 ⁻ⁿ then the negative log is = n
if x > 1 x 10⁻ⁿ then negative log is <n |
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Strong Base pOH calculation
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pOH = - log [SB]
pH = 14 - pOH watch out for diprotic SB in which you will have to double SB concentration - can get pH greater than 14 |
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Calculating pH of weak acids
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ICE table if necessary
--------------------- HA -- ↔ -- A -- H -- initial------------- given -- -- 0 -- 0 -- @ equilibirum-- given-x -- -- x -- x -- -then plug @ equilibrium into Ka equation - ditch -x as it is arbitrary there is no need to reproduce ice table unless there are already initial concentrations -½log(Ka[WA]) |
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Calculating pOH of weak bases
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ICE table if necessary
--------------------- A -- ↔ -- HA -- OH -- initial------------- given -- -- 0 -- 0 -- @ equilibirum-- given-x -- -- x -- x -- -then plug @ equilibrium into Kb equation - ditch -x as it is arbitrary there is no need to reproduce ice table unless there are already initial concentrations -½log(Kb[WB]) |
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Mixing SA & SB
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Determine quantity assuming 100% neutralization
- H⁺ + OH⁻ → H₂O - always exothermic Determine the pH of acid or base - easier to figure out moles remaining and then figure out concetrations Changes in volume are negligble |
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Mixing SA & WB or SB and WA
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HA + OH → H₂O + A⁻
if SA/SB in excess it determines pH of solution use SA/SB tech if WA/WB in equal amount to SA/SB then use WA/WB tech with remaining WA/WB if WA/WB in excess then determine pH of resultant buffer solution |
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Buffers
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Mix of WA/WB conjugate pair
Minimize changes in pH but do not eliminate changes Buffer capacity is the concentrations of acid/base that can be absorbed - [WA] = buffers base capacity - [WB] = buffers acid capacity - Henderson Hasselback eaquation pH = pKa + log (WB/WA) |
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Other uses of Henderson Hasselbach
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if pH and pKa are known then ratio of protonated (WA) to deprotonated (A⁻) ratio
i.e. when pH = pKa ratio in fraction must be 1 to 1 (log 1x10⁰) and mixture must be 50/50 if pH < pKa then acid will be more protonated (WA form) if pH > pKa then acid will be more deprotonated (WB form) for every change of 1 in pH a 9 will be added into the percentage protonated vs. deprotonated i.e. change of 0 pH = 50% change of 1 pH = 90% change of 2 pH = 99% change of 3 pH = 99.9% |
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titrations (general definition)
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-can be used to find concentrations of unknown
-Always use strong acid or strong base as titrant -unknown sample + known titrant |
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Shape of Titration curves
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low to high pH = Strong acid w/ strong base (relatively straight lines)
high to low pH = Strong Base w/ Strong acid (relatively straight lines) Low to high pH curvy = Weak acid w/ strong base High to low pH curvy = weak base w strong acid |
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Equivalence Point
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Equal amounts of titrant and sample (1:1 ratio)
- All reactants (titrant and substance) used up · SB + SA = neutralization · SB + WA = A⁻ (conjugate of WA) in soln · SA + WB = HA (conjugate of WB) in soln - 1:1 ratio - n₁M₁V₁ = n₂M₂V₂ Halfway up vertical component of curve Two equivalence points = diprotice A/B |
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Half Equivalence Point
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half as much titrant than sample (1:2 ratio)
NO point for for SA/SB and SB/SA only works for buffered systems Half equivalence points halfways across horizontal portions of titration curves |
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Indicator
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within 1 unit of the pKa of whatever point you want to indicate
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Strong Acid + Strong Base (or vice versa)
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curve will have very straight lines
equivalence point will be at exactly pH = 7 No half equivalence point |
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Titration of Weak Acid with Strong Base
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equivalence point > 7 (only A⁻ present as HA and titrant neutralize each other)
Buffer region (excess WA vs Titrant) pH determined by buffer system of HA and A⁻ After equivalence point pH determined by amount SB as it is in excess and all HA used up |
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Titration of Weak Base with Strong Acid
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equivalence point < 7 (only HA present as titrant and A⁻ neutralize each other)
Buffer region (excess WB vs Titrant) pH determined by buffer system of A⁻ and HA After equivalence point pH determined by amount SA as it is in excess and all A⁻ used up |
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Weak Diptotic Acid w/ Strong Base
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pH of equivalence point = average of pKa's of half equivalence points (2 of them in diprotic)
- pH = ½(pK₁ + pK₂) (which will be > 7) H₂A + SB → HA⁻ SB → A⁻ |
