Question

Create an ordered list of the tests you will perform on your unknown to identify its cation(Ba^{+2}, Pb^{+2}, Zn^{+2}) and anion(CO_3^{-2},SO_{4}^{-2}, PO_{4}^{-2},SCN^-, Cl^-, NO_3^-). The procedure for each test should be briefly described, along with what a positive and negative result looks like. Assume that each test returns a negative result, except for the last cation and anion test, so that you have to perform all of the tests.

Please help!

Thank you!

Answer 1

cation analysis : Ba⁺², Pb⁺² and Zn⁺²

Test 1: First we will will add HCl to the mixture. It will give white ppt. It may be Ag+ or Pb+2 or Hg+2

So to confirm the presence of Pb+2, we will add hot water. It will dissolve the white ppt indicating the presence of Pb+2. We will take a part of this dissolved ion solution.

confirmation test: we wil add KI to the solution, a yellow ppt will confirm the presence of Pb+2

Pb²⁺ + 2 KI → PbI₂ + 2 K⁺

Test 2: In presence of HCl, on passing H2S no ppt formed

Test 3: we will take the salt in water and then add ammonium chloride and ammonium hydroxide. No ppt forms

Test 4: Pass H₂S gas through the Ammonical solution. A dull white ppt will form showing the presence of Zn+2.

Zn⁺² + H2S ---> ZnS(s) white

The above white ppt will dissolve in dilute HCl ], with evolution of H2S gas (rotten egg smell)

confirmatory test: The presence of Zn+2 can be confirmed by soodium hydroxide test: We will take solution of above test and will add NaOH to it,it will form white ppt of Zn(OH)2. The ppt gets dissolved on addition of excess of NaOH due to formation of Na₂ZnO₂.

Test 5: Take salt solution and add 2g of NH4Cl to it. Boil the mixture and cool it and then add NH₄OH till the solution smells ammonia. Now in this basic solution add (NH4)2CO3 solution. White ppt will be formed. Filter off the ppt and wash with water. Add hot dilute CH3COOH. The white ppt will get dissolved in this hot dilute acetic acid.

confirmatory test for Ba⁺²: to above solution add few drops of potassium chromate solution , a yellow ppt of Barium chromate will be formed which confirms the presence of Ba⁺².

Anion analysis: carbonate, sulphate, phosphate, chloride, thioscynate, nitrate

a) add dilute H2SO4, a brisk effervescence due to evolution of CO2, The gas passed through lime water turns it milky. Confirms presence of carbonate ions

b) Add concentrated H2SO4 to the salt solution, then add AgNO3 to it. formation of ppt indicates presence of halide ion.

Chloride ion forms white ppt with it

Ag+ + Cl- ---: AgCl(s) white

On addition of ammoni the ppt dissolves which confirms the presence of chloride ions

In the same test on addition of conc H2SO4, yellow fumes are formed which shows the presence of nitrate ions due to evolution of NO2.

Nitrate ions can be confirmed by the famous brown ring test. To the salt solution add FeSO4, and then pour conc H2SO4 slowly along the sides of test tube. It will give a distinct brown ring around the wall due to formation of Fe(NO)2+.

c) If the salt solution shows no reaction with H2SO4 it shows the presence of sulphate and phosphate in the salt.

On addition of BaCl2 to the salt solution, a white ppt is formed due to BaSO4, which confirms the presence of sulphate

While, a yello ppt is obtained (which is crystalline) on addition of HNO₃ and ammonium molybdate. It confirms the presence fo phosphate ions.

d)SCN- : These can be analyzed by adding 1mL of conc acetic acid to the salt solution. then add few drops of freshly prepared ferric nitrate solution (Fe(NO₃)₃). A dark red colouration indicates the presnce of SCN-.

the thiocynate ions cannot be tested in presence of phosphate ions. So first we have to remove phosphate ions. This can be acheived by centriguation of ppt. And decanting the supernatant solution. Testing again by addition of (Fe(NO₃)₃). If ppt stops forming then a red colour will be obtained