Q. 41

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 :

Mass of 1 mole of aluminium atom = the molar mass of aluminium = 27 g mol–1

A aluminium atom and ion, differ by three electrons. Aluminium loses three electrons to form aluminium ion.


The mass of one electron = 9.1×10–28 g


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


Therefore, total number of electrons lost = 3


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+ = molar mass of aluminium – mass of electrons


= 27–0.00164 = 26.9984 g mol–1


Therefore, difference in masses of one mole each of aluminium atoms and one mole of its ions


= 27 – 26.9984 = 0.0016 g


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