Question

Solid Aluminum hydroxide decomposes to produce aluminum oxide and water vapor. If 350 mL of water is produced at STP, how many grams of aluminum hydroxide were used? How many grams of aluminum oxide are produced?

Answer 1

2 Al(OH)3   ----------------> Al2O3 + 3 H2O (g)

volume of water produced = 350 mL = 0.350 L

temperature = 273 K

pressure = 1 atm

P V = n R T

1 x 0.350 = n x 0.0821 x 273

n = 0.0156

moles of water = 0.0156 mol

2 Al(OH)3   ----------------> Al2O3 + 3 H2O (g)

from the equation :

2 mol Al(OH)3 --------------> 3 mol H2O

??                ---------------> 00156 mol H2O

moles of Al(OH)3 = 0.0156 x 2 / 3 = 0.0104 mol

mass of Al(OH)3 = 0.0104 x 78

mass of Al(OH)3 used = 0.812 g

2 mol Al(OH)3   -----------> 1 mol Al2O3

0.0104 mol Al(OH)3 ------------->   ??

moles of Al2O3 = 0.0104 x 1 / 2

                        = 5.205 x 10^-3 mol

mass of Al2O3 = 5.205 x 10^-3 x 102

                        = 0.531 g