GRAVIMETRIC DETERMINATION OF THE COMPOSITION OF A MIXTURE OF NaHCO3 AND NaCi BY
ID: 886768 • Letter: G
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GRAVIMETRIC DETERMINATION OF THE COMPOSITION OF A MIXTURE OF NaHCO3 AND NaCi BY THERMAL DECOMPOSITION NaCl is stable in the temperature range between 200 °C and 850 °C, but in that range, NaHCO3 decomposes 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 NaHCO03 originally 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 VALUE (g) 17.8657 18.9142 18.6692 18.6649 18.6604 18.6606 DATA Weight of empty crucible Initial weight of crucible +sample Weight of crucible + residue after 1st heating Weight of crucible + residue after 2nd heating Weight of crucible + residue after 3rd heating Weight of crucible + residue after 4th heating 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. (Use the atomic masses from the back cover of the lab manual Give your answer to 4 significant figures.)Explanation / Answer
2NaHCO3(s) ---> Na2CO3(S) + H2O(g) + CO2(g)
weight of NaHCO3 = 1 gram
mass of gaseous products = 18.9142-18.6606
= 0.2536 grams
that means mass of CO2+H2o = 0.2536 grams
mass ratio of CO2/H2O = 44/18 = 2.4
MASS of H2O =
2*4(H2O)+H2O = 0.2536
= 0.075 grams
mass of CO2 = 0.2536-0.075 = 0.1786 grams
No of moles of CO2 = 0.1786/44= 0.004 mole
No of moles of H2o = 0.075/18 = 0.004 mole
from equation
1mole H2O = 1 mole CO2 = 1 mole Na2co3 = 2 mole NaHCO3
No of moles of NaHCO3 = 0.00406 mole
mass of NaHCO3 decomposed = 0.004*84.007
= 0.3411 grams
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