101 Lab Lecture 3 Percent Composition by Mass Moodle Fall 2010 Compatibility Mode

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?


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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

101 Lab Lecture 3 Percent Composition by Mass Moodle Fall 2010 Compatibility Mode

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