Question

how many phosphorus atoms are there in 0.25moles of P4 molecules

Answer 1

1mole of P₄ molecules contain Avogadro number of P₄ molecules.

so, 1mole of it contains 4 x Avogadro number of P atoms.

0.25 moles of it contains 0.25 x 4 x Avogadro number of P atoms.
                                   
                                     = Avogadro number of P atoms
                                   
                                     = 6.023 x 10²³ Phosphorus atoms