Question

an introduction to chemistry lab

Pre-Lab Study Questions 1. How can the presence of an ion in a solution be detected? ไป dedeed by a lame the ions undec gp color changes By com paring knoum olion o a unKnown vo Can co ure he color ehanges in he lame If a reaction produces an insoluble salt, what will you notice happening in the test tube? 2. So,It wany saヅdissolved and tt a, precipitate oat, solut kan bep eadind on the co lor, ue mig it see a powder or minond sc1l 0 A. Tests for Positive lons (Cations) Unknown number ndet Cation Tested Observations Procedure Observations for Unknown A.1 Flame tests Nat K* A.2 Oxalate test Ca s peset A.3 Ammonium testC osdie A.4 Iron test Fe3+ Identification of the positive ion in the unknown solution From your test results, what positive ion (cation) is present in your unknown?cilcm 0o Explain your choice.

Answer 1

Q.1

The chemical test of known solutions of cation and anions helps us in identifying the cations and anions in the unknown substance because there are some specific tests for cations and anions which are given by specific ions. So we perform to same tests with the unknown sample in order to confirm their presence.

Q.2.

Oxalate test can be performed to find out the compound. CaCl₂ gives positive oxalate test. It gives white precipitate with oxalic acid while Na⁺ does not give this test.

Q.3

Fe³⁺ ions gives blood red colour on reacting with KSCN solution.

Fe³⁺ + KSCN ---> FeSCN²⁺(Deep red in colour)

So the tap water must contain Fe³⁺ ion.

Q.4

Add concentrated NaOH solution to the solution NH₄⁺ ion will librate NH₃ gas which gives dense white fumes when rod dipped in HCl is brought closer to gas. Also that gas turns red litmus paper blue.

NH+4(aq)+OH−(aq) <---> NH3(aq)+H2O(l)

For the PO₄^3- ion test add to the 3-5drops of the unknown, add some nitric acid and ammonium molybdate solution. The phosphate ions give yellow precipitate with that.

PO₄^3-+12 MoO₄^2- (molybdate ion) + 24H⁺ + 3NH₄⁺ ---->  (NH₄)₃[PMo₁₂O₄₀] + 12H₂O

Q.5

Cl^- ions forms precipitate with AgNO₃ and does not dissolve in the HNO₃.

CO₃^2- gives gas with HCl and that is CO₂ gas.

Na⁺ ion gives the bright yellow-orange flame test.

SO₄^2- ions gives ppt with BaCl₂ and does not dissolve in the HNO₃.