A ) What is the pH of a 0.22 M solution of the basic salt sodium formate, NaHCO

Question

A)

What is the pH of a 0.22 M solution of the basic salt sodium formate, NaHCO2?
At 25oC, the Ka of formic acid is 1.8x10-4.

2 points   

B)

Arsenic acid is triprotic. Which of the following reactions is associated with Ka3 for H3AsO4?

1 points   

C)

Rank the following in order from most acidic to most basic (lowest pH to hightest pH). You may need Ka and Kb values from the table in your textbook.
CH3NH, HBr, NaCN, KOH, HOBr

HBr, HOBr, CH3NH2, NaCN, KOH

HOBr, HBr, KOH, CH3NH2, NaCN

HBr, HOBr, NaCN, CH3NH2, KOH

KOH, NaCN, CH3NH2, HOBr, HBr

2 points   

D)

CO2

OH-

HCO3-

H3AsO4 + H2O H2AsO4- + H3O+

H2AsO4- + H2O HAsO4-2 + H3O+

HAsO4-2 + H2O AsO4-3 + H3O+

Explanation / Answer

Solution :- A)What is the pH of a 0.22 M solution of the basic salt sodium formate, NaHCO2?
At 25oC, the Ka of formic acid is 1.8x10-4

Solution :- Salt is given so it will act as base

Therefore lets find the kb

Kb = kw/ ka

Kb= 1*10^-14 / 1.8*10^-4

Kb = 5.56*10^-11

Now lets find the cocnetration of the OH- using the kb value

Kb= [HCOOH][OH-]/[HCOO^-]

5.56*10^-11 = [x][x]/[0.22-x]

Since kb is very small therefore we can neglect the x from the denominator

5.56*10^-11 = [x][x]/[0.22]

5.56*10^-11 * 0.22 = x^2

1.22*10^-11=x^2

Taking square root of both sides we get

3.49*10^-6 = x =[OH-]

Now lets calculate the pOH

pOH= -log [OH-]

pOH= - log [ 3.49*10^-6]

pOH= 5.46

pH + pOH = 14

therefore, pH = 14 – pOH

                           = 14 – 5.46

                          = 8.54

b) Arsenic acid is triprotic. Which of the following reactions is associated with Ka3 for H3AsO4?

Solution :-

Ka3 means the dissociation of the third proton therefore the correct equation is as shown in the option 3

That is          HAsO4-2 + H2O ------- > AsO4-3 + H3O+

C) Rank the following in order from most acidic to most basic (lowest pH to hightest pH).

Solution :-

The order is as follows from lower pH to higher pH

HBr,< HOBr,< NaCN,< CH3NH2,< KOH

Because HBr is the strong acid and KOH is the strong base

D) In the following reaction, which species is acting as a Lewis acid?
CO2 + OH- ----- > HCO3-

Solution :- Lewis acid is the species which accepts the electron pair therefore in the given equation CO2 is accepting the electrons therefore the Lewis acid is CO2

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