Question

A sample of 2.0 moles of a metal reacts completely with excess oxygen to form 62.0 g of M2O. What element is represented by the symbol M?

Answer 1

1) State the chemical reaction

4M + O2 ---> 2M2O

2) State the molar ratios

4 mol M : 1 mol O2 : 2 mol M2O

3) Calculate the number of moles of M2O produced

4 mol M / 2 mol M2O = 2 mol M / x => x = 2 mol M * 2 mol M2O / 4 mol M

=> x = 1 mol M2O

4) Calculate the molar mass of M2O

molar mass = mass in grams / number of moles = 62.0 g / 1 mol = 62.0 g/mol

5) Calculate the atomic mass of M

molar mass of M2O = 2 * atomic mass of M + atomic mass of O =>

62.0 = 2 * atomic mass of M + 16

=> atomic mass of M = [62.0 - 16] / 2 = 23

6) Compare that atomic mass with the atomic masses of the metals in the periodic table.

You will find that 23 is the atomic mass of sodium.

Answer: sodium