Silica (also known as silicon dioxide) reacts with hydrofluoric acid to produce silicon tetrafluoride and water vapor.
a) Write the balanced chemical equation for the reaction

b) 12.5 g of silicon dioxide is reacted with a 24.6 g of hydrofluoric acid. What is the theoretical yield of water vapor?

c) If the actual yield of water vapor is 2.50 g, what is the percentage yield of the reaction?

The problem is solved by balancing the reaction equation as shown below. The stoichiometry of the reaction is now applied in solving the question.

The balanced reaction equation is:

SiO2 + 6HF → H2[SiF6] + 2H2O

Using stoichiometry

The amount of silicon dioxide reacted = 12.5 g/60.08 g/mol = 0.21 mole

1 mole SiO2 yields 2 moles of H2O

0.21 mole of SiO2 yields 0.21 * 2/1 = 0.42 moles of H2O

The amount of HF reacted = 24.6 g/20.01 g/mol = 1.23 moles of HF

6 moles of HF yield 2 mole of H2O

1.23 moles of HF yields 2 * 1.23/6 = 0.41 moles of H2O

Hence HF is the limiting reactant.

Theoretical yield of H2O = 0.41 moles of H2O * 18 g/mol = 7.38 g

The percentage yield of the water is = actual yield/theoretical yield * 100

Hence;

% yield = 2.50 g/7.38 g * 100 = 33.9%

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