GRAVIMETRIC DETERMINATION OF THE COMPOSITION OF A MIXTURE OF NaHCO 3 AND NaCl BY

Question

GRAVIMETRIC DETERMINATION OF THE COMPOSITION OF A MIXTURE OF NaHCO3 AND NaCl BY THERMAL DECOMPOSITION

NaCl is stable in the temperature range between 200 oC and 850 oC, but in that range, NaHCO3decomposes by the reaction:

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

If a mixture of the two is heated to temperatures in the above range, the gaseous products will escape and the residue will contain the original NaCl and an amount of solid Na2CO3. The Na2CO3 is related stoichiometrically to the amount of NaHCO3 which has decomposed. If the reaction has proceeded to completion (no remaining NaHCO3), the moles, and therefore the weight of NaHCO3originally in the sample, are able to be determined.

We heat the sample several times, weighing the residue after each heating.

We use the following criterion to establish that the reaction is complete:

The reaction is considered complete if the change in weight of the residue

is less that 5.0 mg (plus or minus) in two successive heatings.

The following data were collected in such a determination:

Some heatings may have been unnecessary. You may assume that the weight after the 4th heating is the final weight, even if prior heatings were unnecessary.

From the data, answer the following questions:

From the equation for the decomposition of NaHCO3, what is the weight in grams lost when 1.000 g of NaHCO3 is decomposed thermally

Explanation / Answer

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

when 1 g of NaHCO3 is thermally decomposed, according to the stoichiometric coefficients, 0.5 g of solid Na2CO3 will be left behind with NaCl.

So, weight in grams lost=0.5 g

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