Chemistry --> reaction

Question

Hello experts,

i would like to ask how can i find the mas of CaS with the moles of calcium by knowing the mass of reagents?

Reaction: 2C + Na2SO4 + CaCO3 --> Na2CO3 + CaS + 2 CO2

The mass of CaCo3 is 5,01g and the mass of Na2SO4 is 4,27g

I dont understand why i find the answer correct by using only the moles of S in Na2SO4 and NOT by using the moles of Ca in CaCO3 ........ (ANSWER mass CaS = 2,17g)

Any help would be appreciated

Answer 1

You don't use CaCO3 because it is in excess and the Na2SO4 is the limiting reactant (SEE BELOW).

2C + Na₂SO₄ + CaCO₃ --> Na₂CO₃ + CaS + 2CO₂

molar mass Na2SO4 = 142 g / mol

molar mass CaCO3 = 100. g / mol

molar mass CaS = 72.1 g / mol

Find the limiting reactant. One way is to simply divide moles of each reactant by the respective coefficient in the balanced equation. Whichever answer is lower represents the limiting reactant.

For Na2SO4: 4.27 g x 1 mol / 142 g = 0.0300 mols (÷1->0.03)

For CaCO3: 5.01 g x 1 mol / 100 g = 0.0501 mols (÷1->0.05)

Since 0.03 is less than 0.05, Na2SO4 is LIMITING and this is why you use it instead of CaCO3 to find the amount of product formed.

Mass CaS formed = 0.0300 mols Na2SO4 x 1 mol CaS / mol Na2SO4 x 72.1 g CaS/mol = 2.16 g CaS