CHM2046L Unknown 2

Topics: Solubility, Acid, Carbon dioxide Pages: 3 (746 words) Published: September 24, 2013
Unknown Scheme:
You can use this scheme as a guide when you are making your own. Don't just show up to class with this though.


Unknown Scheme: 2

I. Description
A) Phase, solubility, color, odor, shape

II. Flame Test
A) Orange flame  Na+ present (possibly K+, NH4+)
B) Red flame  Ca2+ present (no Na+)
C) Purple flame  K+ present (possibly NH4+)
D) No change in flame color  Ca2+, K+, Na+ absent (possibly NH4 present)

III. pH Test
A) Add a little bit of unknown to a small amount of neutral water B) Fill several vials with test solution and use indicators to determine pH

Common pHs for Anions and Cations:
pH 1.5: HSO4- present; CO32-, HCO3-, OH- not present
pH 2.5: SO42- present
pH 3-3.5: Al(H2O)63+ present; CO32-, HCO3-, OH- not present
pH 5-6: NH4+ and/or Mg(H2O)62+ present
pH 7: Cl- and/or NO3- and/or SO42- and/or K+ and/or Na+ and/or Ca2+ present pH 9-10: HCO3- present; HSO4-, OH-, and Al(H2O)63+ not present pH 10-12: CO32- present; HSO4-, Al(H2O)63+, NH4+ not present pH 13: OH- present; NH4+, Al(H2O)63+, HSO4-, HCO3- not present

If unknown is an insoluble compound, use the liquid to test pH pH 7: CaSO4*2H2O
pH 9.5: MgCO3
pH 10: CaCO3
pH 10.5: Mg(OH)2
pH 12.5: Ca(OH)2

*Separations are necessary to test for Cations. To Separate, add excess 3-6M NH3+ to separate Mg+ and Al2+ from Na+, K+, NH4+, and Ca2+*

*Ca2+ Separation:
A) Treat sample solution with excess 0.5M Na2CO3
B) After ppt. has formed, centrifuge in test tube
C) Treat centrifuged sample with more Na2CO3 and repeat until Ca2+ is out

A) If no ppt. forms after addition of NH3, then Mg(H2O)62+ is not present B) If ppt. forms with NH3, but doesn’t form a ppt. with the addition of excess 2M NaOH, then Mg(H2O)62+ is present and Al(H2O)3+ is absent

IV. NH4+ Test
A) Observe test results for pH test (if acidic, NH4+ could be present) B) Place about 2-5mL of sample solution in vial and add an equal...
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