Question

Solid iron(III) hydroxide decomposes to produce

iron(III) oxide and water vapor. 0.930 L
of water vapor is produced at STP. How much
iron(III) hydroxide was used?
Answer in units of g.

Answer 1

1) write down the balance equation

2Fe(OH)₃ ----> Fe₂O₃ + 3H₂O

2) when you are at STP, there is a constant conversion factor--> 22.4 L= 1 mol of gas. using this conversion, we can convert the Liters to moles of water vapor

0.930 Liter (1 mol H₂O/ 22.4 Liters)= 0.0415 mol H₂O

3) convert from moles of water to moles in question, Fe(OH)₃ using the moles-to-moles

0.0415 mol H₂O (2 mol Fe(OH)₃/ 3 mol H₂O)= 0.0277 mol Fe(OH)₃

4) finally, convert moles to grams using the molar mass of Fe(OH)₃.

molar mass of Fe(OH)₃= 55.8 + (3 x 16.0)+ ( 3 x 1.01)= 106.8 g/mol

0.0277 mol Fe(OH)₃ (106.8 g/ 1 mol)= 2.96 grams