Chemistry 122 Spring 2 Page 4 of 9 Exam II Multiple Choice Section: question is

Question

Chemistry 122 Spring 2 Page 4 of 9 Exam II Multiple Choice Section: question is worth 2 points and has only one possible ans place your in the box. 1. Which one of the following mixtures would result in a buffered solution when 1.0 L of each the two solutions are mixed? a. 0.1 M KOH and 0.1 M NH4Cl b. 1 M KOH and 0.2 M NH4Cl 0.2 M KoH and 0.2 M NHCI D. 2 d. 0.1 M KOH and 0.2 M NH 0.2 M KoH and 0.1 M NHCI 2. Which one of the following best describes Le Chatelier's Principle? a. Once a reaction reaches equilibrium, the reaction will stay at equilibrium forever. b. Once a reaction has stopped, the reaction at equilihrium is

Explanation / Answer

Buffer means mixture of weak acid and its salt or mixture of weak base and its salt.

for eg acetic acid is a weak acid , so mixture of acetic acid and any acetate salt will behave as Buffer.

Ammonium Hydroxide is a weak base so mixture of ammonium hydroxide and any ammonium salt will behave as Buffer.

1) KOH + NH4Cl ---> NH4OH + KCl

a) 0.1 M KOH + 0.1 M NH4Cl ----> 0.1 M NH4OH + 0.1 M KCl

After completion of reaction , resulting solution contains only 0.1 M NH4OH , there is no NH4Cl hence it can not act as buffer.

b) 0.1 M KOH + 0.2 M NH4Cl ----> 0.1 M NH4OH + 0.1 M KCl + 0.1 M NH4Cl ( unreacted)

After completion of reaction , resulting solution contains both 0.1 M NH4Cl and  0.1 M NH4OH , hence it can act as buffer.

c) 0.2 M KOH + 0.2 M NH4Cl ----> 0.2 M NH4OH + 0.1 M KCl   ( No unreacted NH4Cl)

d) KOH and NH3 both are bases , So there will be no reaction , mixture of two bases will not as a buffer.

e) 0.2 M KOH + 0.1 M NH4Cl ----> 0.1 M NH4OH + 0.1 M KCl + 0.1 M KOH ( unreacted) mixture of two bases will not as a buffer.

Therefore Answer option B).

3) HF is weak acid , NaF is its salt.

a) 1 HF +1 NaF ---> no reaction     ( Buffer)

b) 1 NaF + 0.5 NaOH ---> no reaction ( Not buffer, it is a mixture of strong base and its salt)

c) 1 HF + 0.5 NaOH ---> 0.5 NaF + 0.5 HF + H2O ( buffer)

d) 1 NaF + 0.5 HCl ---> 0.5 HF + 0.5 NaF + 0.5 NaCl ( Buffer)

Except option B) all the resulting solutions have mixture of NaF and HF so except option B all are buffer.

Answer that combination would not form buffer mixture is option B

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