The Moles and The Avogadro Constant

Question

Another large-scale industrial process is the production of quicklime (calcium oxide, CaO) from limestone by heating in a lime kiln.

a. What is the equation for the thermal decomposition of limestone?

b. Using your equation, calculate how many tonnes of quicklime (calcium oxide, CaO) would be produced from 1 tonne of limestone. (Ar: Ca = 40)

Answer 1

Thermal decomposition of limestone (CaCO₃) is...

CaCO₃(s) ==> CaO(s) + CO₂(g)

In words, this is....

calcium carbonate (limestone) decomposes to calcium oxide (quicklime) plus carbon dioxide.

In order to determine the mass (in tonnes) of quicklime (CaO) formed from 1 tonne of limestone (CaCO3), we will first have to convert tonnes to grams to moles, and then at the end, convert from moles back to tonnes. This will not involve the use of Avogadro's constant. The conversion factors we'll use are...

1 tonne (metric ton) = 1000 kg = 1x10⁶ g

molar mass CaCO₃ = (40 g + 12 g + 3*16 g) = 100. g

molar mass CaO = (40 g + 16 g) = 56 g

(1 tonne CaCO₃)(1x10⁶ g / tonne)(1 mole CaCO₃ / 100 g) = 10,000 moles CaCO₃ used

Next, use the stoichiometry of the balanced equation to find moles CaO produced...

10,000 moles CaCO₃ x 1 mol CaO / 1 mol CaCO₃ x 56 g CaO / mol CaO = 560,000 g CaO

Finally convert this back to tonnes...

560,000 g x 1 tonne / 10⁶ g = 0.56 tonnes of CaO produced