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SUSB-052 - Gravimetric Analysis of a Mixture

 

1. Preliminaries

In this exercise, you weigh a sample of a mixture of NaHCO3 and NaCl, heat it to decompose the NaHCO3 and weigh the product to determine the composition of the mixture. The NaCl is unaffected by the heating. Any changes in weight are due to NaHCO3 alone.

 

2. Stoichiometry:

The stoichiometric relationship between the reagents is given by the overall equation:

2NaHCO3(s)   =   Na2CO3(s) + H2O(g) + CO2(g)

This equation implies that:

2 moles of NaHCO3  react to produce 1 mole of water and 1 mole of CO2 or,

1 mole of NaHCO3  react to produce 1/2 mole of water and 1/2 mole of CO2 or,

84.01 grams of NaHCO3  react to produce 18.01/2 grams of water and 44.01/2 grams of CO2 or,

84.01/84.01 = 1.000 gram of NaHCO3  reacts to produce 18.01/2*84.01 = 0.1072 grams of water and 44.01/2*84.01 = 0.2619 grams of CO2

1.000 gram of NaHCO3  reacts to produce (0.1072 + 0.2619 = 0.3691) grams of gaseous products

I.e., weight loss is directly proportional to the initial weight of NaHCO3.

weight loss = 0.3691 * weight of NaHCO3 or,

weight of NaHCO3 = weight loss / 0.3691

Or, rephrasing the above, consider what happens when a 1.0000 g sample of pure NaHCO3 is heated to decomposition - a temperature high enough to vaporize water. ( CO2 is certainly gaseous as well. )

1.0000 g of NaHCO3 = 1000.0 mg = 1000.0 mg / 84.01 mg/mmol = 11.90 mmol

When it decomposes, it will produce 1/2 X 11.90 = 5.950 mmol of gaseous H2O and 5.950 mmol of CO2

These will weigh, respectively,

5.950 mmol X 18.01 mg/mmol H2O = 107.2 mg and,

5.950 mmol X 44.01 mg/mmol CO2 = 261.9 mg

So, 1.0000 g of NaHCO3 will lose 107.2 + 261.9 = 369.1 mg = 0.3691 g when it decomposes.

If we know the original and final weight of a sample of NaHCO3, the loss in weight will be proportional to the weight of NaHCO3 in the sample, with a proportionality constant of 0.3691 . I.e., again we have as above,

weight loss in heating (g) = 0.3691 X weight of NaHCO3 (g)

 

3. A Sample Calculation

Suppose a 0.8643 g sample of an unknown weighs 0.6098 g after heating. The weight loss is 0.8643 - 0.6098 = 0.2545 g.

This corresponds to 0.2545 / 0.3691 = 0.6895 g of NaHCO3, and the sample must have contained

100 X 0.6895 / 0.8643 = 79.78 % NaHCO3

 


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Last Update: RFS 2015-08-27