Question

1. Determine the pH when 5.89 g of sodium acetate (NaOOCCH₃) is dissolved in 190 mL of water. Ka of acetic acid is 1.8 10^-5

                                    NaOOCCH₃Na⁺   +  ^-OOCCH₃

                                    ^-OOCCH₃  +  H₂OHOOCCH₃  +   OH^-

2. A student wants to kill Dr. Dahm, so he walks buys 10 grams of potassium cyanate (KCN) and drops all 10 grams into Dr. Dahms pepsi can.  Determine the pH if the can holds 335 mL of liquid and the Ka of HCN is 6.2 10^-10.

                                    KCN   K⁺ +  CN^-

                                    CN^-   +  H₂O    HCN +  OH^-

Answer 1

1)CH3COONa is salt of weak acid CH3COOH and NaOH . for such salts

pH = 1/2 [pKw + pKa + log C]

pKa = - log Ka = - log [1.8 x 10^-5] = 4.74

C = (W / MW) (1000 / V in mL)

C = [5.89 / 82.03] [1000 / 190]

C = 0.38 M

pH = 1/2 [14 + 4.74 + log 0.38]

pH = 9.16

2) KCN is salt of weak acid HCN and strong base KOH

for such salts

pH = 1/2 [pKw + pKa + log C]

pKa = - log Ka = - log [6.2 x 10^-10] = 9.21

C = ( W /MW) (1000 / V in mL)

C = (10 / 65.12) (1000 / 335)

C = 0.46 M

pH = 1/2 [14 + 9.21 + log 0.46]

pH = 11.44