Calculate the number of moles and molecules in 19.86 g. of Pb(NO3)2. [Pb = 207, N = 14, O = 16]

(i) Gram molecular mass of Pb(NO₃)₂

= 207 + (2 × 14) + (6 × 16)

= 207 + 28 + 96 = 331

Number of moles = 19.86/331

= $0.06\text{ moles}$

Hence, the number of moles in 19.86 g. of Pb(NO₃)₂ = 0.06 moles

(ii) 331 g  or 1 mole of Pb(NO₃)₂ contains = 6.023 × 10²³ molecules

⇒ 0.06 mole of Pb(NO₃)₂ contains = 0.06× 6.022 × 10²³ molecules

Hence, number of molecules in 19.86 g. of Pb(NO₃)₂ = 0.06 × 6.023 × 10²³ moles