Question

How many molecules of white phosphorus (P4) are contained in 248 grams of the substance. (P=31)

Answer 1

Molar mass of P4 = 4*31 = 124

Now,
no of moles = Given mass / Molar mass
= 248/124 = 2 moles

Number of molecules = Avogadro number* Number of moles
= 2*6.022*10^23 = 12.044*10^23 molecules

Answer 2

248 g of phosphorus = 248/31 = 8 moles

if 1 mole of phosphorus = 6.022×10^23

∴ 8 moles of phosphorus = 8×(6.022×10^23)

                                           =4.8176×10^24