Experiment #5 – Separation of a Mixture
Laboratory Overview
CHEM 1361
September 2012
Gary S. Buckley, Ph.D.
Department of Physical Sciences
Cameron University
Learning Objectives
•Quantitatively determine the percentage composition of a mixture
•Use atomic weight relationships to determine and use the percentage composition
of chemical compounds
•Set up, ignite, and optimize a Bunsen burner
Table of Contents
(you may click on any of the topics below to go directly to that topic)
•Overview
•Characteristics of a Hydrate
•Steps in Separation of a Mixture
•Calculational Steps
Overview
A mixture consisting of magnesium sulfate heptahydrate (also known as
Epsom salt), SiO2 (sand), and NaCl (table salt) will be separated and the
quantities of each component will be determined.
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Steps in Separation of Mixture
The initial mixture contains MgSO4•7H2O, SiO2, and NaCl. The goal is to find
the percentage of each.
The initial step in the process is heating the mixture with a Bunsen burner. The
MgSO4•7H2O is dehydrated losing its water while neither the SiO2 nor the NaCl
is affected by the heating. Thus after heating, the initial mixture consists of
MgSO4 (no longer with water), SiO2, and NaCl.
The addition of water to these three remaining compounds will result in the
dissolution of MgSO4 and NaCl with only SiO2 remaining in the dish.
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Characteristics of a Hydrate
The first step in the separation process relies on the dehydration of a
substance known as a hydrate. A hydrate is an ionic chemical compound
which contains a fixed amount of water in the spaces between the ions.
The example of a hydrate used today is Epsom salt, MgSO4•7H2O. When
heated a hydrate loses its water according to the reaction (Δ above the
arrow indicates heating):
Δ
MgSO4 7H 2O(s) 
 MgSO4 (s) + 7 H 2O(g)
1  246 g
1 120 g
7 18 g
The masses below each compound are the molar masses of each
compound multiplied by the coefficient in the balanced equation. Based
on the mass relationships , every 246 g of MgSO4•7H2O dehydrated will
release 126 g of H2O. If half that amount of MgSO4•7H2O is dehydrated,
only half of 126 g of H2O will be given off, etc. Thus, by measuring the
weight loss upon heating, the amount of MgSO4•7H2O initially present
may be determined.
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Experimental Notes
The picture to the right depicts the basic experimental
setup .
Since this is the first experiment in which you will use a
Bunsen burner, a little instruction is in order. The flexible
tubing goes to a natural gas outlet. Though burners can
be different, in the one pictured there is an air
adjustment collar as indicted in the figure. This can be
moved to let in more or less air. To light the burner
initially, have the air collar nearly closed, turn on the gas
at the outlet, and use the striker to light the flame. If the
flame is yellow, there is too little air – open the air collar
more. A good flame is blue and will have an outer cone
and an inner cone – the tip of the inner cone is the
hottest part of the flame.
Air Collar
Be aware of lit Bunsen burners – the flames are nearly
invisible but quite hot.
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Calculational Steps
The mass of hydrate present initially will be determined by the difference
between the starting mass of the mixture and the mass after the initial heating
step. Based on the relationship on a previous slide:
mass MgSO 4 •7H 2O present initially = mass H 2 O lost ×
246g MgSO 4 •7H 2 O
126 g H 2 O
After adding water and drying, only SiO2 will be left in the evaporating dish
allowing the determination of its mass in the original mixture.
The mass of NaCl in the initial mixture is determined by the difference in mass
between the original mixture, the mass of hydrate determined in the original
mixture and the mass of sand in the original mixture.
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NaCl
MgSO4
SiO2
NaCl
Add Water
SiO2
Heat
Original Mixture
MgSO4·7H2O
SiO2
The overall separation scheme may be represented
by the figure above (also in your lab book).
End of presentation