Question

1. Does the pH increase, decrease or remain the same on the addition of the of the following?

a.) LiF to an HF solution b.)KI to an HI solution c.)NH4Cl to an NH3 solution d.) HBr to an CH3COO-/CH3COOH (acetic acid/acetate buffer) solution

2. Which of the following (select all that apply) gives a buffer solution when equal volumes of the two solutions are mixed?

a.)		0.10 M NH3 and 0.10 M NH4Cl
b.)		0.10 M NH3 and 0.10 M NaOH
c.)		0.10 M NH4Cl and 0.10 M NaOH
d.)		0.50 M NH4Cl and 0.25 M NOH

Answer 1

1. Does the pH increase, decrease or remain the same on the addition of the of the following?

a.) LiF to an HF solution

pH will increase, since HF is a weak acid, when F- is aded ( LiF --> Li+ +F-) there will be addition of a conjugate base, F-, which shift the equilibrium toward more HF and less H+ in solution, pH increases as H+ decreases

b.)KI to an HI solution

No effect, KI and HI remain as K+ , I- , H+ in solution

c.)NH4Cl to an NH3 solution

The pH will DECREASE, since NH4Cl --> NH4+ + Cl-

notethat NH4+ is acidic, since NH4+ -- > NH3 + H+

this will decrease pH

d.) HBr to an CH3COO-/CH3COOH (acetic acid/acetate buffer) solution

HBR --> Strong acid, will dissociate as H+ and Br-

the buffer is acidic, so more HBr in solution will increase H+ in solution, so pH decreases

2. Which of the following (select all that apply) gives a buffer solution when equal volumes of the two solutions are mixed?

A buffer is any type of substance that will resist pH change when H+ or OH- is added.

This is typically achieved with equilibrium equations. Both type of buffer will resist both type of additions.

When a weak acid and its conjugate base are added, they will form a buffer

The equations:

The Weak acid equilibrium:

HA(aq) <-> H+(aq) + A-(aq)

Weak acid = HA(aq)

Conjugate base = A-(aq)

Neutralization of H+ ions:

A-(aq) + H+(aq) <-> HA(aq); in this case, HA is formed, H+ is neutralized as well as A-, the conjugate

Neutralization of OH- ions:

HA(aq) + OH-(aq) <-> H2O(l) + A-(aq) ; in this case; A- is formed, OH- is neutralized as well as HA.

Now,

For the weak base equilibrium:

B(aq) + H2O(l) <-> BH+(aq) + OH-(aq)

Weak base = B;

Conjugate acid = BH+

Neutralization of OH- ions:

BH+(aq) + OH-(aq) <-> B(aq) + H2O(l); in this case, OH- is neutralized by BH+, as well as B is created

Neutralization of H+ ions:

B(aq) + H+(aq) <-> BH+(aq)

a.) 0.10 M NH3 and 0.10 M NH4Cl

This is a buffer, since NH3 and NH4+ are present in equal ratio, so conjugate acid + weak base arr presnet

b.) 0.10 M NH3 and 0.10 M NaOH

weak base + strong base = no buffer

c.) 0.10 M NH4Cl and 0.10 M NaOH

this is neutralization

NH4+ + OH- = NH3 + H2O

this is no buffer

d.) 0.50 M NH4Cl and 0.25 M NOH

this is a buffer

0.25 M of NH4+ react with 0.25 M of NaOH f rotm NH3 and H2O

NH4+ is left and NH3 has just formed

this is a buffer!