Question

A 0.157−mol sample of a metal oxide,

M₂O₃

, weighs 23.5 g.

(a) How many moles of O are in the sample?

(b) How many grams of M are in the sample?

(c) Which element is represented by the symbol M? (Give the elemental symbol of that element.)

Answer 1

a) One mole of M2O3 contains 3 moles of O atoms

Hence 0.157 mole of M2O3 will contain 3 * 0.157 moles of O atoms

Number of moles of O atoms = 0.471 moles

b) Molar mass of M2O3 = Mass/number of moles = 23.5/0.157 = 149.68 gm/mol

Let the molar mass of metal be M

2M + 3 * 16 = 149.68

2M = 101.68

M = 50.84 gm/mol

grams of M in the sample = 101.68/149.68 * 23.5 = 15.963 gms

c) The element can be Chromium (Cr), since it can exists in +3 oxidation state and also has molar mass closer to the chromium