Question

Compute the difference in masses of one mole each of aluminium atoms and one mole of its ions. 

(Mass of an electron is 9.1×10^–28 g). Which one is heavier?

Answer 1

Mass of 1 mole of aluminium atom = the molar mass of aluminium 

= 27 g mol^–1 

An aluminium atom needs to lose three electrons to become an ion, Al³⁺ 

For one mole of A1³⁺ ion, three moles of electrons are to be lost. 

The mass of three moles of electrons = 3 × (9.1×10^–28 ) × 6.022×10²³ g 

                                                            = 27.3 × 6.022 ×10^–5 g 

                                                            = 164.400 ×10^–5 g 

                                                            = 0.00164 g 

               Molar mass of Al³⁺ 

                                                            = (27–0.00164) g mol^–1 

                                                            = 26.9984 g mol^–1 

                                          Difference = 27 – 26.9984 

                                                           = 0.0016 g