8/9/2019 101 Lab Lecture 3 Percent Composition by Mass Moodle Fall 2010 Compatibility Mode
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Percent Composition by Mass
.
2. Percent Composition by Mass3. Hydrates
4. Ex erimental Procedure
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Purpose
Part I1. To determine the percent composition of sodium
hydrochloric acid HCl.
theoretical value.
3. To calculate the percent error in the experimentaldetermination.
Part II
1. To determine the percent composition by mass of
2. To identify the unknown salt
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Percent Composition
Thepercent composition by mass is the percentby mass of each element in a compound.
Theoretical %Composition of an element =
nx molar mass of element x 100%
Ex erimental %Com osition of an element
mass of elementin gramsx 100%
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Empirical Formula and Percent Composition
Compounds having the same empirical formula willhave the same ercent com osition et the are
distinctly different compounds
Formula % C % H Molar mass (g/mol)
CH
(empirical)
92.3% 7.7% 13.02(empirical)
2 2
(acetylene)
. . . .
(benzene)
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Two measurements are used to determineex erimental ercent com osition of a
compound.
1. The mass of the original sample
. e mass o epro uc.
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1.45 g of iron is reacted with oxygen to yield 1.92 g
of iron oxide.
1 What is the experimental percent
.
of iron oxide?
8/9/2019 101 Lab Lecture 3 Percent Composition by Mass Moodle Fall 2010 Compatibility Mode
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= .
Mass of oxygen = 0.47g .
%Fe = 1.45 g Fe x 100 = 75.5%. g
%O = 0.47g O x 100 = 24%
1.92g
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.
Molar mass of FeO = 55.84g Fe + 16.00g O
= . g mo
%Fe = 55.84 g Fe x 100 = 77.73 %71.84 g
= = . .71.84 g
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Experimental Procedure Part I
Na2CO3 + 2HCl 2NaCl + CO2 + H2O
Weigh clean dry small beaker using theanalytical balance
Add around 1g of sodium carbonate and weigh
Add dilute HCl slowly while shaking, continue
becomes clear.
(Avoid excess HCl)Evaporate the water
Dry in an oven for half an hour
oo an we g .
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Hydrates
H drates are com ounds that have a s ecificnumber of water
Formulas of hydrates are written by first writing the formula formolecules attached to them.
e an y rous compoun an en a ng a o o owe y e
numberof water molecules present.
BaCl22H2O barium chloride dihydrate
4 2
Sr(NO3)2 4H2O
strontium nitrate tetrahydrate
Water in the hydrate is called water of hydration or water of
crystallization , it can be removed by heating
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Many natural and commercial salts are hydrated salts.
indicates that 6 moles of water of hydration are present for each
mo e o o 2 sa .
The bond is so weak that simply heating the hydrated salt willbreak the bond and liberate the water molecules as vapor.
,number of water molecules associated with each hydrate can b
determined.
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Identification of an unknown hydrate
Your unknown will be one of the hydrates listed in the
.
You will be determine experimentally the mass of waterlost from the hydrate after heating at high temperature
accor ng o e o ow ng genera reac on:
salt x H2O salt + x H2OHeat
,x can be determined.
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Experimental Procedure Part II
Weigh a clean and dry crucible with lid
Heat on a Bunsen burner or hot plateKeep the lid offset to allow water vapor
o escape rom e ea e y ra e
Allow to cool and weigh
Heat again , cool and reweigh until
.Caution! Use tongs to carry the crucible
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