Question

Assume your "impurity" is oxygen in the form of Al₂O₃ on the surface of your metal (Aluminum). Assuming all Al is accounted for (0.056 g), calculate the number of moles of Al₂O₃ present. Assume 2.7% impurity.

Answer 1

Calculate mass of oxygen

Mass of oxygen = mass of al Al x 2.7/100

0.056*2.7/100=0.001512 g O

Calculate moles of Oxygen

Mol = mass / molar mass

Molar mass of O is 15.999 g/mol

therefore

0.001512/15.998=9.45 x 10^-5 mol O

2 mol Al needs 3 mol O therefore

Moles of Al with 9.45 x 10^-5 mol O =   9.45 x 10^-5 mol x 2 mol Al/ 3 mol O

=6.3 x 10^-5 mol Al

So moles of Al associated with moles of O are 6.3 x 10^-5 mol

Therefore moles of Al2O3 = 6.3 x 10^-5 mol