Acid/Base Equilibrium - Buffers 1. If a solution of ammonia/ammonium buffer cont

Question

Acid/Base Equilibrium - Buffers

1. If a solution of ammonia/ammonium buffer contains 0.229 M NH3, calculate the molarity of NH4Cl needed if the solution is to have a pH of 8.19. ( Kw=10-14, Kb=1.80×10-5 )

NH3(aq) + H2O(l) = NH4+(aq) + OH-(aq)

_____________________M

2. A solution of hydrofluoric acid/fluoride buffer has a molar ratio of KF to HF of 8.04. Calculate the pH of the solution. Round your answer to 3 significant digits. ( Kw=10-14, Ka=6.80×10-4 )

HF(aq) = F-(aq) + H+(aq)

3. A solution of formic acid/formate buffer contains 0.160 M HCO2H and 2.15 M HCO2Rb. Calculate the pH of the solution. Round your answer to 3 significant digits. ( Kw=10-14, Ka=1.70×10-4)

HCO2H(aq) = HCO2-(aq) + H+(aq)

1. If a solution of ammonia/ammonium buffer contains 0.229 M NH3, calculate the molarity of NH4Cl needed if the solution is to have a pH of 8.19. ( Kw=10-14, Kb=1.80×10-5 )

NH3(aq) + H2O(l) = NH4+(aq) + OH-(aq)

_____________________M

2. A solution of hydrofluoric acid/fluoride buffer has a molar ratio of KF to HF of 8.04. Calculate the pH of the solution. Round your answer to 3 significant digits. ( Kw=10-14, Ka=6.80×10-4 )

HF(aq) = F-(aq) + H+(aq)

3. A solution of formic acid/formate buffer contains 0.160 M HCO2H and 2.15 M HCO2Rb. Calculate the pH of the solution. Round your answer to 3 significant digits. ( Kw=10-14, Ka=1.70×10-4)

HCO2H(aq) = HCO2-(aq) + H+(aq)

Explanation / Answer

Acid/Base Equilibrium - Buffers

Henderson-Hasselbach equation for buffer solutin
Basic buffer: pOH = pKb + Log([salt]/[base])
Acid buffer: pH = pKa + Log([salt]/[acid])

1. If a solution of ammonia/ammonium buffer contains 0.229 M NH3, calculate the molarity of NH4Cl needed

if the solution is to have a pH of 8.19. ( Kw=10-14, Kb=1.80×10-5 )

NH3(aq) + H2O(l) = NH4+(aq) + OH-(aq)

Basic buffer: pOH = pKb + Log([salt]/[base])
pOH = 14 - pH = 14 - 8.19 = 5.81
pKb = -Log(Kb) = -Log(1.80×10-5) = 4.75
[base] = [NH3] = 0.229 M
[salt] = [NH4Cl]
pOH = pKb + Log([NH4Cl]/[NH3])
Log([NH4Cl]/[NH3]) = pOH - pKb = 5.81-4.75= 1.06
[NH4Cl]/[NH3] = 11.6
[NH4Cl] = 11.6*[NH3] = 11.6*0.229 = 2.66 M

2. A solution of hydrofluoric acid/fluoride buffer has a molar ratio of KF to HF of 8.04. Calculate the

pH of the solution. Round your answer to 3 significant digits. ( Kw=10-14, Ka=6.80×10-4 )

HF(aq) = F-(aq) + H+(aq)
Acid buffer: pH = pKa + Log([salt]/[acid])
pKa = -Log(Ka) = -Log(6.80×10-4) = 3.2
[salt]/[acid] = [KF]/[HF] = 8.04
pH = pKa + Log([salt]/[acid])
pH = 3.2 + Log(8.04) = 4.1

3. A solution of formic acid/formate buffer contains 0.160 M HCO2H and 2.15 M HCO2Rb. Calculate the pH

of the solution. Round your answer to 3 significant digits. ( Kw=10-14, Ka=1.70×10-4)

HCO2H(aq) = HCO2-(aq) + H+(aq)
Acid buffer: pH = pKa + Log([salt]/[acid])
pKa = -Log(Ka) = -Log(1.7×10-4) = 3.77
[salt]=[HCO2Rb] = 2.15 M
[acid] = [HCO2H] = 0.16 M
pH = pKa + Log(2.15/0.16)
pH = 3.77 + Log(8.04) = 4.9

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