# Acid Base and Buffer

6.1 Calculate [H3O+] and pH in a 0.05M HCN-solution (Ka=5.85E-10).

Solution:
Step 1) Insert equation of dissociation: HCN + H2O > CN- + H3O+
Step 2) Insert (Ka) 5.85E-10
Step 3) Insert [HCN] = 0.05 M
Step 4) Calculate pH and [H3O+]

6.2 Calculate [H3O+] and pH pH in a 0.05M acetic acid solution (Ka=1.75E-5). Is acetic acid a stronger acid than HCN ?

Solution:
Step 1) Insert equation of dissociation: CH3COOH + H2O > CH3COO- + H3O+
Step 2) Insert (Ka) = 1.75E-5
Step 3) Insert [CH3COOH] = 0.05 M
Step 4) Calculate pH and [H3O+]
Step 5) Compare acid strength (acetic acid and HCN).

6.3 Calculate Ka for a 0.1 M solution of C6H5COOH (benzo acid). ([H3O+] = 0.00248 M)

Solution:
Step 1) Insert equation of dissociation: C6H5COOH + H2O > C6H5COO- + H3O+
Step 2) Insert 0.1 in [C6H5COOH] field and 0.00248 in [H3O+] field
Step 3) Calculate 6.4 Calculate Ka for a 0.5 M solution of HCOOH (pH = 2.02)

6.4 Calculate Ka in 0.5M HCOOH   (Equlibrium: pH = 2.02).
Solution:
Step 1) Insert equation of dissociation: HCOOH + H2O > HCOO- + H3O+
Step 2) Insert O.5 in [HCOOH] field and 2.02 in pH field
Step 3) Calculate

6.5 Determine [HF] in a pH = 2 solution (Ka=6.94E-4).

Solution:
Step 1) Insert equation of dissociation: HF + H2O > F- + H3O+
Step 2) Insert (Ka)= 6.94E-4
Step 3) Insert pH = 2
Step 4) Calculate
Step 5) Solution: [HF(Before dissoc.)] = [H3O+] + [HF(After dissoc.)].

6.6 Calculate [OH-] and pH if [NH3] = 0.05 M (Kb=1.8E-5).

Solution:
Step 1) Insert equation of dissociation: NH3 + H2O > NH4+ + OH-
Step 2) Insert (Kb) = 1.8E-5
Step 3) Insert [NH3] = 0.05 M
Step 4) Calculate pH and [OH-]

6.7 Calculate pH in a 0.1M NH4Cl solution (KNH4+=5.6E-10).

Solution:
NH4Cl > NH4+ + Cl-
Step 1) Insert equation of dissociation: NH4+ + H2O > NH3 + H3O+
Step 2) Insert KNH4+ = 5.6E-10
Step 3) Insert [NH4+] = 0.1 M
Step 4) Calculate pH

6.8 Calculate [OH-] and pH in a 1 dm3 [NH3] = 0.2 M (Kb=1.8E-5). Decide pH if 2.675 grams of NH4Cl is added?

Solution:
Step 1) Insert equation of dissociation: NH3 + H2O > NH4+ + OH-
Step 2) Insert (Kb) = 1.8E-5
Step 3) Insert [NH3] = 0.2 M
Step 4) Calculate pH and [OH-]
Step 5) Calculate (convert) 2.675 g NH4Cl to mole using the molecular calculator: 2.675gNH4Cl = 0.05 mol, in 1 dm3 -> 0.05 M
Step 6)Insert Kb, [NH3] and [NH4+] (common ion field) from NH4Cl
Step 7) Calculate pH

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