Question

Table 8.1. Mass data for analysis of alum. Mass of weighing paper for alum (g) C. 3933 Mass of weighing paper plus alum sample (g) L 333 Mass of empty heated and cooled crucible (g) Mass of crucible plus Baso after firstheating ogy Mass oferucible plus Basorafter 3TIST second heating Mass of crucible plus mass of paper 1. ISO Baso, after third heating (g) BaSoy (s) Instructor's initials CALCULATIONS Table 8.2. Percent soi in alum Mass of alum (g) Mass of Baso, (g)b Percent so in your alum sample Theoretical percent SO n your pure alum sample Analytical purity of your alum sample (%o EXPERIMENT 8

Answer 1

1. % age SO₄^2-by mass in dry alum

Molar mass of KAl(SO₄)₂ is 258.2050 g/mol

Sulfur	32.0	2 (no. of atoms)
Oxygen	15.9	8(no. of atoms)

Molar mass of (SO₄)₂ is 192.1252 g/mol

% age SO₄^2-by mass in dry alum = Molar mass of (SO₄)^2-/ Molar mass of KAl(SO₄)₂ * 100

(192.1252/258.2050)*100 = 74.40%

% age SO42-by mass in wet alum

Molar mass of KAl(SO₄)₂*12H₂O is 474.3884 g/mol

Molar mass of (SO₄)₂ is 192.1252 g/mol

% age SO₄^2-by mass in wet alum = (Molar mass of (SO₄)^2-/ Molar mass of KAl(SO₄)₂ * 12 H₂O )*100

192.1252/474.3884 = 40.499 %

So, we see that there is decrease in (SO₄)^2-% by mass in wet alum

2. The qualitative test for sulfate uses the insolubility of barium sulfate (BaSO₄). When an aqueous solution of a barium salt (usually BaCl₂) is mixed with an aqueous solution containing sulfate, a white precipitate of insoluble BaSO₄ forms according to the net ionic equation: Ba²⁺ (aq) + SO₄^2- (aq) → BaSO₄ (s). A positive test for SO₄^2-, therefore, is the observation of a white precipitate when an aqueous BaCl₂ solution is mixed with the aqueous test solution.

As precipitate contains sulfate from BaSO₄, Ba²⁺ and SO₄^2- from Ba CO₃ only Ba²⁺

Total SO₄^2- = 192.1252 g/mol

Total weight of precipitate = Molar mass of BaSO₄+ molar mass of Ba from BaCO₃ = 192.12+137.32 = 329.44

% age SO₄^2-by mass in precipitate = 192.12/329.44 = 58.3%

So, we see that there is decrease in (SO₄)^2-% by mass

3. The decanted liquid if was cloudy, that means some more crystals can be precipitated out of it. This second crop of crystals may also be collected, but if you choose to collect these crystals, they should be kept separate from the main crop. It is always good laboratory practice to keep different crops of crystals separate until the identity and purity of each crop is determined.

second crops almost always contain more impurities than the first crop and the time needed to purify them sometimes far outweighs the additional yield. There might be increase in the percent SO₄^2- by mass