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Experiment 7
Gooch filter crucible or fritted-glass funnel
Mother liquor- liquid from which a substance precipitates or crystallizes Filtrate- liquid that passes through the filter
Slurry- suspension of solid in liquid
Calcium ion can be analyzed by precipitation with oxalate in basic solution to form CaC2O4.H2O The precipitate is soluble in acidic solution because the oxalate anion is a weak base CaC2O4.H2O (calcium oxalate monohydrate ; MW = 146 g/mol)

Equation: Ca2+(aq) + C2O42-(aq) CaC2O4.H2O Large, easily filtered, relatively pure crystals of product will be obtained if the precipitation is carried out slowly, which can be done by dissolving Ca2+ and C2O42- in acidic solution & gradually increasing the pH by thermal decomposition of urea, Co(NH2)2. Equation : Co(NH2)2 + 3H2OCO2 + 2NH4+ + 2OH-

Methyl red indicator: [below pH 4.8 – red ; above pH 6.0 - yellow]

Sample experimental Data

Funnel #1
Funnel#2
Average
Mass of glass funnel, initial (grams)
35.2653
34.5269
34.8961
Mass of glass funnel + ppt. after drying (grams)
35.6653
34.9569
35.3111
Mass of product (grams)
0.4000
0.4300
0.4150
Moles of product corresponding to moles CaC2O4.H2O
2.74 x 10-3
2.94 x 10-3
2.84x10-3
% Ca2+ in the unknown sample
0.4384 = 0.44
0.4704 = 0.47
0.45

Calculations : moles of product = moles CaC2O4.H2O (mw = 146 g/mol) Funnel 1:

Funnel 2:

% Ca2+ in the unknown sample : (atomic wt. Ca = 40 g/mol)
% Ca2+ (w/v) =
Funnel 1:
% Ca2+ =
=0.4384 = 0.44%

Funnel 2:
% Ca2+ =
= 0.4704 =0.47%
Problem Solving:
The Ca in a 200.0 ml sample of natural H2O was determined by precipitating cation as CaC2O4. The precipitate was filtered, washed, and ignited in a crucible with an empty mass of 26.6002 g. The mass of the crucible plus CaO was 26.7134 g. a) What is the mass of Cao?

b) How many moles of Ca are present?
= = 2.0 x 10-3 mol Ca
c) What is the concentration of Ca in H2O in units of...
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