Question

what is the number of molecules present in 0.25 moles of oxygen?

Answer 1

in 0.25 moles there are 1.505*10^23 molecules (1 mole of any element=6.023*10^ 23 molecules).

Answer 2

1 mole contains 6.022*10^23 molecules
0.25 moles contain 6.022*10^23 * 25/100
                                =6.022*10^25*25
150.55*10^25 molecules