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GRAVIMETRIC ANALYSIS
OF A TWO COMPONENT MIXTURE

PURPOSE
The purpose of the experiment is to determine the percent composition by mass of a two component mixture made up of NaHCO3 and Na2CO3.

INTRODUCTION
Mixtures are composed of two or more components, which do not react with each other. Stoichiometry deals with mass relationships between reactants and products in chemical reactions. It is based on balanced equations and the mole concept. In this experiment, the concepts of stoichiometry will be used to calculate the percent composition of a mixture composed of sodium hydrogen carbonate and sodium carbonate. The mass of both reactants and the mass of both products will be calculated using only experimental mass measurements. This is called a gravimetric analysis.

The percent composition of this mixture can be calculated from mass data by knowing the following five factors.
 1.  The total mass of the mixture used for analysis. 2. The identity of common products to which both components can be converted. 3.  The two balanced chemical equations that describe the reactions used to convert each component to the common products. 4.  The mass of one of the products from the above reactions. 5.  The identity of the components of the mixture.

A mixture of NaHCO3 and Na2CO3 reacts with hydrochloric acid solution to produce three common products, sodium chloride, carbon dioxide and water according to the following balanced chemical equations.

NaHCO3(s) + HCl(aq) à NaCl(s) + CO2(g) H2O(l) (1)

Na2CO3(s) + 2HCl(aq) à 2NaCl(s) + CO2(g) + H2O(l) (2)

After boiling off the water and any unreacted excess acid, the remaining residue is solid which can easily be dried and weighed. Its mass can then be used to calculate the percent composition of the mixture. Since the mixture contains two components of unknown mass, two equations must be used to determine their mass.

The two simultaneous equations, which will be used to calculate the masses of each component in the mixture, must have identical unknown quantities. The first important mass relationship equates the mass of the mixture to the masses of the two components.

Mass of mixture (gr) = mass (gr) of NaHCO3 + mass (gr) of Na2CO3 (3)

The second important mass relationship is based on the mass of the solid residue, NaCl.

Mass of NaCl (gr) = mass (gr) of NaCl formed + mass (gr) of NaCl formed (4) from the NaHCO3 from the Na2CO3

These two equations result in not two unknowns but four unknowns. However, the mass of NaHCO3 can be related to the mass of NaCl formed form the NaHCO3 and the mass of Na2CO3 can be related to the mass of NaCl formed from the Na2CO3.

These relationships can be stated using the stoichiometric elements obtained from the reactions given in equations (1) and (2). The important product related to your mass measurements is NaCl. Equations (1) and (2) show that one mole of NaHCO3 produces only one mole of NaCl whereas one mole of Na2CO3 gives two moles of NaCl. Using the techniques developed in the section of the your text on stoichiometry you can write two equations. The first equation you need to develop is to correlate the mass of sodium chloride formed from sodium bicarbonate in terms of the mass of NaHCO3 the original mixture. The second equation you need is to correlate the mass of sodium chloride formed from sodium carbonate in terms of the mass of NaCO3 the original mixture. After this is completed, the two equations (3) and (4) will be expressed in only two unknowns.

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DATA SHEET

PROCEDURE
Support a clean crucible and cover on a wire triangle and heat for 5 minutes until they are thoroughly dry. Allow them to cool to room temperature and weigh on an analytical balance. Handle the crucible with tongs.

Transfer approximately 0.50 g of the unknown mixture to the crucible and weigh the crucible, cover and contents on the analytical balance. Carefully measure 5 mL of dilute HCl into a graduated cylinder under the HOOD. CAUTION: HCl is very corrosive. Avoid getting it on your skin or clothing. If you do, wash it with water immediately. Working under the hood, use a plastic pipette to add the HCl one-drop at a time to the mixture in the crucible. Wait for the reaction to subside before adding the next drop of acid. This avoids spattering and loss of material.

After all the acid has been added, heat gently to drive off the excess acid and water. This procedure should be performed in the exhaust hood. It is best to wave the flame under the crucible and to keep the cover partially open. The rate of heating must be gentle enough to prevent the acid from boiling and thus spattering the mixture. When the solid that is formed appears dry, heat the crucible in the full flame of the burner for about 10 minutes. Allow the crucible and its contents to cool to room temperature, then weigh on the analytical balance.

To ensure that all volatile substances have evaporated, heat again for 5 minutes, cool and reweigh. This weight should agree with the previous one within 0.001 g.

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