Analytical Gravimetric
Determination
Suh Kwon
Purpose
• To measure the number of a given
substance in a solution by
precipitation, filtration, drying, and
weighing
– Approximating the amount of SO42- ions in a
sample of alum, KAl(SO4)2· 12H20
Background Information
• Gravimetric analysis is used to
determine the amount of a
substance by finding its mass, and
then using the mass to find the
quantity of the substance
– Example: to find the measurement of solids
suspended in a water sample, a known volume
of water is filtered and the collected solids are
weighed
For this experiment…
• The precipitate that will be formed in this
experiment is barium sulfate, BaSO4
– Forms very fine crystals
• Beforehand, calculate how
much 0.200 M Ba(NO3)2 you
would need to totally
precipitate all of the sulfate
ion present in solution
http://www.jinyakc.com/jy/My%20Pictures/9.jpg
!!!
• Make sure you add twice the calculated
amount of Ba(NO3)2 because for every 1
mole of alum, there are 2 moles of Ba(NO3)2
KAl(SO4)2· 12H2O + 2Ba(NO3)2
 KNO3 + Al(NO3) 3 + 2BaSO4 + 12H2O
Materials
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Buchner Funnel
A piece of Whatman No. 42 filter paper
Filter flask
1.059 g of alum compounds
50.0 mL of distilled water
22.3 mL (calculated) of 0.200 M Ba(NO3)2
Heating Plate
Analytical Balance
Drying oven set at 50°C
Filter Flask
An analytical
balance is an
instrument that's used
to measure mass at a
very high degree of
precision.
Analytical Balance
http://www.usdoj.gov/dea/photos/lab/analytic
al_balance_mettler_ae-260.jpg
http://www.laboratoryequipmentworld.com/gifs/filter-flasks.jpg
Filter Paper
Buchner Funnel
http://upload.wikimedia.org/wikipedia/commons/5/52/Buchner_funnel.jpg
http://www.isss.biz/prodImages/Whatman-Filter-Paper-Grade-2.jpg
http://www.rickly.com/sai/images/BUCHNER.JPG
Procedures
1. Dissolve the 1.059 g of alum in the 50.0 mL
of distilled water
2. Add twice the amount of 0.200 M Ba(NO3)2
to alum solution, stirring constantly
3. Heat this solution for 15 minutes and allow
the solution to stand overnight
Next Day: You will discover that fine crystals
had appeared in the solution
Procedures (2)
4. Weigh the filter paper separately and
determine its mass on the analytical
balance
5. Filter the solution through the Buchner
funnel, containing the filter paper to
collect the crystals
6. Remove the filter paper and allow it to dry
in the 50°C drying oven
7. Weigh the paper and crystals together and
determine their total mass
http://www.creativechemistry.org.uk/activities/images/buchner.gif
http://depts.washington.edu/chem/courses/labs/162labs/images/PA230280.JPG
Data
INITIAL
• Mass of alum: 1.059 g
• Volume of distilled water: 50.0 mL
• Amount of 0.200 M Ba(NO3)2 : 22.3 mL
COLLECTED
• Mass of filter paper (alone): 1.675 g
• Mass of filter paper + filtered crystals: 2.715 g
Equations
• Balanced Chemical Formula:
*** KAl(SO4)2· 12H2O + 2Ba(NO3)2
 KNO3 + Al(NO3) 3 + 2BaSO4 + 12H2O
*** Ba2+ + SO42-  BaSO4 (s)
• % Sulfate = (Mass of Sulfate/Mass of Sample) * 100%
• Percent Error = (Theoretical Value – Actual Value) * 100%
Theoretical Value
Calculations
• How much 0.200 M Ba(NO3)2 would be needed to
totally precipitate all of the sulfate ion present
in the alum solution:
1.059 g alum · 1 mol of alum · 2 mol Ba(NO3)2 · 1 L of Ba(NO3)2 · 1000 mL = 22.3 mL
474.46 g alum 1 mol of alum 0.200 mol Ba(NO3)2 1 L
• The percent of sulfate ion in the alum based
upon the experiment:
2.715 g filter paper + barium sulfate
- 1.675 g paper
1.040 g BaSO4
1
2
1.040 g BaSO4 · 1 mol BaSO4· 1 mol SO42-· 96.04 g SO42- = 0.4281 g SO42233.37 g BaSO4 1 mol BaSO4 1 mol SO423
% sulfate = 0.4281 g SO42- · 100% = 40.43%
1.059 g alum
• The theoretical % of sulfate ion in the alum:
192.08 g SO42- · 100% = 40.48 %
474.46 g alum
• The percent error in this investigation:
% error = 40.43 – 40.48 · 100% = -0.15%
40.48
Conclusion
• With the help of this experiment, you were
able to find the amount of sulfate ions in a
sample of alum compound, comparing that
value to the theoretical amount necessary
• A negative percent error signifies that
there was a production of a solid
throughout the reaction