Question

The lanthanides(elements 57-71) react with oxygen. A sample of lanthanide combines with oxygen to form a pure compound containing 14.63% oxygen. Which lanthanide is this , and what is the emporical formula of the compound?

Answer 1

Assume that the lanthanide is denoted by 'L'

The oxide formed upon reaction with oxygen is: L₂O₃

Assume that 100 grams of this compound is formed

Mass of O = 14.63% = 14.63 grams

Moles of O = 14.63/16 = 0.914375

1 mole of the compound contains 3 moles of O.

So, moles of compound present = 0.914375/3 = 0.30479167

1 mole of the compound contains 2 moles of L.

So, moles of L present = 0.30479167*2 = 0.60958334

Mass of lanthanide = 100-14.63 = 85.37 g

So,

MW of L = Mass/moles = 85.37/0.60958334 = 140.046 g

So, the identity of 'L' is Cerium.

Empirical formula is Ce₂O₃.

Hope this helps !