Stoichiometery Questions :- 1 ) Quicklime, CaO(s), is prepared from limestone by

Question

Stoichiometery Questions :- 1) Quicklime, CaO(s), is prepared from limestone by roasting it. The reaction is: CaCO3(s) CaO(s) + CO2(g). What mass (in t) of quicklime can be prepared from 1.000 Mg of limestone that contains 94.6% CaCO3?

2) A 7.78 g sample of a mixture of NaCl and NaHCO3 is heated to 300°C. The NaHCO3 decomposes as shown, but the NaCl is stable (i.e., does not react): 2NaHCO3(s) Na2CO3(s) + H2O(g) + CO2(g) After decomposition, the sample weighs 5.93 g. Calculate the percent by mass of NaCl in the original mixture.

Explanation / Answer

1) From the given reaction we can see that 1 mole of CaO is produced starting with 1 mole of CaCO3

mass of CaCO3 = 1 mg

it has 94.6% CaCO3 = 0.946 x 1 = 0.946 mg

moles of CaCO3 = mg/molar mass = 0.946/100.09 = 9.45 mmol

mass og CaO formed = 9.45 x 56.08 = 0.53 mg

2) From the reaction we can see that, 2 moles of NaHCO3 decompose to give 1 mole of Na2CO3

loss of mass = 7.78 - 5.93 = 1.85 g

Mass fraction of,

CO2 = 44.01/(44.01+18.02) = 0.71

So in 1.85 we have = 1.85 x 0.71 = 1.3135 g

mols = 1.3135/44.01 = 0.030 mols of CO2

So mols of Na2CO3 = 0.030 mols

mass of Na2CO3 = 3.1797 g

moles of NaHCO3 in original sample = 2 x 0.030 = 0.060 mols

mass of NaHCO3 = 0.06 x 84.0066 = 5.04 g

mass NaCl in sample = 7.78-5.04 = 2.74 g

So mass percent of NaCl = (2.74/7.78) x 100 = 35.22 %

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