If you have 30.0g of SiO2 reacting with 200.0mL of 3.00M HF and 13.2g of SiF4 actually forms, can you answer the following: a) what mass of SiF4 should form? b) what mass of SiO2 is left unreacted?  C) what is the percentage yield of SiF4? 

Expert Answers

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A) In order to calculate the maximum mass of SiF4 that can form, also called the theoretical yield, we need to find out which of the two reactants is the limiting reactant:

`SiO_2 + 4HF -> SiF_4 + 2H_2O` 

The molar masses we will need to know are:

SiO2: 60.1 g/mol

SiF4: 104 g/mol

moles SiO2 = 30.0g X (1 mol/60.1g) = 0.499 moles

moles HF = (200.0 ml)(1 Liter/1000 ml)(3.00 moles/Liter) = 0.600 moles

The mole ratio of HF to SiO2 is 4:1 according to the balanced equation. We don't have four times as many moles of HF as SiO2, therefore HF is the limiting reactant and it determines the amount of SiF4 that can be produced:

0.600 mol HF X (1 mol SiF4/4 mol HF) X (104g/1mol) = 15.6 g SiF4 that can be produced.

B) The mass of SiO2 that reacted is:

0.600 mol HF X (1 SiO2/4 HF) X (60.1g/1mol) = 9.00 grams

30.0g SiO2 - 9.00g reacted = 21.0 grams SiO2 left unreacted

C) Percent error = actual yield/theoretical yield X 100%

 = 13.4g/15.6g = 0.84 X 100% =  84.6% yield

 

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