What is the theoretical yield of H2SiF6? What is the percent yield?

Question

Hydrofluoric acid solutions cannot be stored in glass containers because HF reacts readily with silica dioxide in glass to produce hexafluorosilicic acid (H₂SiF₆).

SiO₂(s) + 6 HF(aq)  H₂SiF₆(aq) + 2 H₂O(l)

42.0 g SiO₂ and 69.0 g of HF react to yield 45.8 g H₂SiF₆.

Answer 1

SiO₂(s) + 6 HF(aq)  H₂SiF₆(aq) + 2 H₂O(l) .. balanced equation

Since we are given the amounts of BOTH reactants, the first thing we must do is to determine which reactant may be in limiting supply. One easy way to do this is to simply divide moles of each reactant by the corresponding coefficient in the balanced equation. Whichever value is less will represent the limiting reactant.

Molar mass SiO₂ = 60.1 g / mole

Molar mass HF = 20.0 g / mole

Molar mass H₂SiF₆ = 144.1 g / mole

Find limiting reactant:

For SiO₂: 42.0 g x 1 mol / 60.1 g = 0.6988 moles (÷1 -> 0.699)

For HF: 69.0 g x 1 mol / 20.0 g = 3.450 moles (÷6 -> 0.575)

Since 0.575 is less than 0.699, HF is LIMITING and moles of HF will be used to determine theoretical yield

Theoretical yield of H₂SiF₆:

3.450 mols HF x 1 mol H₂SiF₆ / 6 mol HF x 144.1 g H₂SiF₆ / mol = 82.9 g H₂SiF₆ (theoretical yield)

Percent yield:

% yield = actual yield / theoretical yield (x100%)

% yield = 45.8 g / 82.9 g (x100%) = 55.2% yield

(be sure to check all of the math)