Mass of 1 mole of phosphorus(P₄) = 124g

So 124 g of phosphorus contains 6x10²³ molecules

P₄ molecule contains 4 atoms

So 124g of phosphorus contains =4x 6 x 10²³ atoms

So 12.4g of phosphorus contains = 4 x 6 x 10²³ x 12.4 = 2.4x10²³ atoms

124

Mass of 1 mole of phosphorus = 124g

So moles in 12.4g phosphorus = 124 = 10 moles

12.4