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Question

Consider a buffered solution containing CH3NH3Cl and CH3NH2. Whihc of the following statements concerning this solution is true? (Ka for CH3NH3=2.3*10^(-11):

a) A solution consisting of 0.10 M CH3NH3Cl and 0.10 M CH3NH2 would have a higher buffering capacity than one containing 1.0 M CH3NH3Cl and 1.0 M CH3NH2.

b) If [CH3NH2]>[CH3NH3], then the pH is larger than the pKa value.

c) Adding more [CH3NH3Cl] to the initial buffer solution will decrease the pH.

d) If [CH3NH2]<CH3NH3], then the pH<3.36

e) If [CH3NH2]=[CH3NH3], then the pH=10.64

Answer 1

Step 1:

The buffered solution contains and .

of the is .

Definition of pH:

.

(a) Case(i) :

Buffer capacity, β, is a quantitative measure of the resistance of a buffer solution to pH change on addition of hydroxide ions

If the concentration of and are equal then .

A solution containing 0.10 M of and 0.10 M of has .

A solution containing 1.0 M of and 1.0 M of has .

Buffering capacity depends on amount of base added.

A solution containing 0.10 M of and 0.10 M of has less buffering capacity when compared to solution containing 1.0 M of and 1.0 M of .

The statement is False.

Step 2:

(b) Case (ii):

If then ratio of , then pH is smaller than .

The statement is False.

Comments

Contd....

Step 3:

(c) Case(iii):

Adding to the initial buffer solution would make the ratio resulting decrease in pH.

The statement is True.

Step 4:

(d) Case(iv):

If , then pH will increase.

The statement is False.

Step 5:

(e) Case(v):

If then

The statement is true.

Solution:

The statement (c) and (e) are true.