Question

Topic : Sampling, Gravimetry, Equilibrium, Titrations

1/ Fusion (dissolving in hot, molten inorganic flux) is applied for the following reason:

(A) The analyte is an organic substance that is not soluble in acids.

(B) The analyte is an inorganic substance that is not soluble in acids.

(C) The analyzed material has a lot of impurities.

(D) The analyzed material contains volatiles.

(E) Interferences have to be removed from the sample.

2/ Wet ashing is a sample preparation procedure in which:

(A) High temperature is used to decompose organic substances.

(B) The sample is dissolved in water.

(C) A sample is oxidized by means of H2O2 or HNO3.

(D) Metals are dissolved in appropriate organic solvent.

(E) It is used mainly for analysis of organic compounds.

3/ Which of the following is not a desirable property of a gravimetric precipitate?

(A) Should be obtained in a pure form

(B) Must be insoluble in the media used for precipitation

(C) Should consist of small particles

(D) Must be easily filterable

(E) Should possess a known composition

4/ Which of the following statements is true if SO3 is removed from the reaction mixture?

2 SO2 (g) + O2 (g) ⇌ 2 SO3 (g)

(A) The equilibrium shifts left.

(B) The amount of SO2 will decrease.

(C) The amount of O2 will increase.

(D) No changes will be observed.

(E) The pressure will increase.

4/ Homogeneous precipitation means:

(A) Two homogeneous solutions are mixed.

(B) No precipitate is formed in the solution.

(C) The product is continuously removed from the mixture.

(D) The precipitating agent is generated in the analyte solution.

(E) The reaction takes place at elevated temperature to keep everything in solution.

5/ Back titration is a procedure in which:

(A) Titrant is added to analyte until the reaction is complete.

(B) The weight of the titrant is measured instead of its volume.

(C) Excess standard reagent is added to the analyte and the former is titrated with a second standard reagent.

(D) The reaction product is titrated with a second standard reagent.

(E) Excess of analyte is titrated with a second standard reagent.

Answer 1

Answer 1):- Option A) is correct - " The analyte is an organic substance that is not soluble in acids." In fusion, mixture of salt and organic substance is heated to a temperature above the melting point of salt. This result in the formation of water soluble salts of organic substances.

Answer 2):- option C) is correct - " a sample is oxidised by means of H2O2 or HNO3.

Answer 3):- option C) is correct - " should consist of small particles" is not a desirable property of a gravimetric precipitate. Generally, precipitate consisting of large particles are preferred in gravimetric work because large particles are easy to filter .

Answer 4):- option B) is correct: " the amount of SO2 will decrease". According to Le- Chatelier principle, reducing the concentration of product (SO3) will shifts the equilibrium towards right side ( product side) and thus, concentration of reactant (SO2) decreases.

Answer 5):- Option (D) is correct one."Precipitating agent is generated in the analyte solution"- in homogenous precipitation.

Answer 6):- Option C) is correct-" Excess standard reagent is added to the analyte and the former is titrated with second standard reagent". Back titration is also called indirect titration.