Question

21. The pH of a 0.120 M phenol solution is (Ka of phenol = 1.3 x 10-10):

a. 1.000

b. 10.81

c. 11.11

d. 5.40

e. 4.48

22. Which list gives equal concentrations of the following solutions in order of decreasing acidity?

HC₃H₅O₃ (K_a = 1.4 x 10^-4)       H₂O (K_a = 1.0 x 10^-14)              HOCl (K_a = 3.5 x 10^-8)

HCN (K_a = 4.9 x 10^-10)                HCl (K_a = 2 x 10⁶)

a. HC₃H₅O₃ > HCl > HOCl > HCN > H₂O

b. HCl > HC₃H₅O₃ > HOCl > HCN > H₂O

c. HCN > HOCl > HCl > HC₃H₅O₃ > H₂O

d. HCl > HC₃H₅O₃ > HOCl > H₂O > HCN

e. H₂O > HCN > HOCl > HC₃H₅O₃ > HCl

23. Which statement is true?

a. As the strength of a weak acid increases, the percent dissociation decreases

b. As the concentration of a weak acid increases, the percent dissociation increases.

c. As the strength of a weak acid decreases, the percent dissociation increases.

d. The percent dissociation of a weak acid is independent of the identity of the weak    

      acid.

e. As the value of K_a increases, the percent dissociation increases.

25. Which of the following salts will form a basic solution?

NaBr

Ca(NO₃)₂

NH₄Cl

NaCH₃CO₂

Al(ClO₄)₃

Answer 1

Answer – 21) We are given, [C₆H₅OH] = 0.120 M , Ka = 1.30*10^-10

We need to put ICE chart -

C₆H₅OH + H₂O ------> H₃O⁺ + C₆H₅O^-

I 0.120                          0           0

C -x                              +x          +x

E 0.120-x                      +x          +x

Ka = [H₃O⁺] [C₆H₅O^-] / [C₆H₅OH]

1.30*10^-10 = x*x /(0.120-x)

1.30*10^-10 *(0.120-x) = x²

We can neglect x in the 0.120-x , since 5% rule

2.15*10^-6 = x²

x = 3.95*10^-6 M

so, x = [H₃O⁺] = 3.95*10^-6 M

so, pH = -log [H₃O⁺]

           = -log 3.95*10^-6 M

           = 5.40

So answer for this question is d) 5.40

22) When we are given equal concentrations of the following solutions in order of decreasing acidity. We know as the Ka value decreasing the acidity decreasing, so list of order of decreasing acidity as follow –

b. HCl > HC₃H₅O₃ > HOCl > HCN > H₂O

23) In this one we are given statements and we need to choose the correct one.

a statement is false, since as the strength of a weak acid increases, the percent dissociation increase not decreasing.

b statement is false, since as the strength of a weak acid increases, the percent dissociation increases not concentration increase

c statement is false, since as the strength of a weak acid increase not decreases, the percent dissociation increases.

d statement is also false, since as the percent dissociation of a weak acid is dependent of the identity of the weak acid.

e statement is true, as the value of K_a increases, the percent dissociation increases.

25 ) We know the salt when soluble and form the strong base then that salt is form a basic solution

NaBr + H₂O ----> NaOH + HBr  

In this one both are strong acid and strong base, so this form neutral solution.

Ca(NO₃)₂ + H₂O ----> Ca(OH)₂ + 2HNO₃  

In this one both are strong acid and strong base, so this form neutral solution.

NaCH₃COO + H₂O ----> NaOH + CH₃COOH  

In this one there is weak acid and strong base, so this form basic solution.

Al(ClO₄)₃ + H₂O ----> Al(OH)₃ + 3 HClO₄

In this one there is strong acid and weak base, so this form basic solution.

So answer for this one is - NaCH₃CO₂