ANAL 367: The Chemistry and Qualitative Analysis of Anions Post-Laboratory Quest

Question

ANAL 367: The Chemistry and Qualitative Analysis of Anions Post-Laboratory Questions (Use the spaces provided for the answers and additional paper if necessary) 1. Suppose that a solution is tested for NO2 lon by 4. A solution ls known to contain Cr, NOs NO2 adding 3M H2SO4 and heating, repeating this and Soions. Which of these lons can be positively process until no further reaction occurs. This solution identified? Describe the reactions used and the is then analyzed for NO3 ion by adding FeSO4 solu results. tion and more H2SO4, heating the test tube and its contents. The NO3 ion reaction is positive. In this case, is it accurate to state that NO3 ion was present in the original solution? Briefly explain. Write appro- priate equations to support your answer. 2. A solution contains a mixture of CI and Br ions. Can both be positively identified? Briefly explain. Write appropriate equations to support your answer. 5. Why wouldn't HCl be a good acid to use to deter- mine the solubility of a silver precipitate in acid solution? 3. The solubility product constants of AgCI, AgBr, and Agl are, respectively, 1.7 × 10-10 4.1 × 10-13 and 1.5 x 10-16. The Ksp expression for each silver halide, AgX, is: When the ion product, [Ag 1X,exceeds the Kp Agx(s) will precipitate untilthe ion product once again equals Ksp If the Ag ion concentration in the mixture is 0.067M (diluted from 0.1M), calculate the detec- tion limit of this reaction for each halide, the value be- low which there will be no observed reaction.

Explanation / Answer

1.

H2SO4 + 2 NO3- — HNO3 + SO4–2

This reaction converts the nitrate into nitric acid. By heating the pure reactants in a distillation apparatus, very pure (fuming) nitric acid can be obtained.

FeIISO4 + 4 HNVO3 FeIII(NO3)3 + H2SO4 + NIVO2 + H2O

2.When chlorine (as a gas or dissolved in water) is added to sodium bromide solution, the chlorine takes the place of the bromine. Because chlorine is more reactive than bromine, it displaces bromine from sodium bromide

As an ionic equation (ignoring the ‘spectator’ potassium ions):

Br2 + 2I- I2 + 2Br-

We can see that the bromine has gained electrons, so it has been reduced. The iodide ions have lost electrons, so they have been oxidised.

3.AgCl=1.7X10-10

AgBr=4.1X10-13

AgI=1.5X10-16

Ksp=[Ag][X]

(1.7X10-10 )(0.067)

(4.1X10-13)(0.067)

(1.5X10-16  )(0.067)

The value whichever is low would be the suitable halide

4 (i) To a solution of the suspected sulfate add dilute hydrochloric and a few drops of barium chloride/ nitrate solution.
(ii) Add lead(II) nitrate solution.

(i) A white precipitate of barium sulfate.
(ii) A white precipitate of lead(II) sulphate.

Test (i) is more definitive.

(i) Ba2+(aq) + SO42–(aq) ==> BaSO4(s)
Any soluble barium salt + any soluble sulphate forms a white dense barium sulphate precipitate.

(ii) Pb2+(aq) + SO42–(aq) ==> PbSO4(s)

Neither white precipitate is soluble in excess hydrochloric acid.

5. As silver chloride also precipitates

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