So there are three formulas that you must be aware of hen attempting these questions (by technicality, only two because the third is a transposition of the second).
These formulas are :
mole x L = # of atoms (where L = Avogadro's Constant)
mass / Mr = mole (where Mr = Molar Mass)
Mole x Mr = mass
So no we can easily ace these questions
1) a. 98.3 g of Hg
mass of Hg = 98.3g
Mr " " = 201 g / mol
mole = [tex] \frac{98.3 g}{201 g / mol} [/tex]
= 0.489 mol
∴ # of atoms = 0.489 mol * (6.03 * 10²³)
= 2.95 * 10²³ atoms
c. 10.7g of Li
mass of Li = 10.7 g
Mr " " = 7 g / mol
thus mole = [tex] \frac{10.7g}{7 g / mol} [/tex]
= 1.529 mol
# of atoms = 1.529 mol * (6.03 * 10²³)
= 9.22 * 10²³
2) a. 6.84 g of C₁₂H₂₂O₁₁
mass of sucrose = 6.84 g
Mr " " = ∑ (Mr of each element * amount of each element's atom)
= ∑ [(12 * 12)+(1 * 12)+(16 * 11)] g / mol
= 342 g / mol
∴ mole of sucrose = [tex] \frac{6.84 g}{342 g / mol} [/tex]
= 0.02 mol
c) 68.0g of NH₃
mass of NH₃ = 68.0 g
Mr " " = [(14 * 1) + ( 1 * 3)] g / mol
∴ mole of NH₃ = [tex] \frac{68.0 g }{17 g / mol} [/tex]
= 4 mol
3) a. 3.52 mol of Si
mole of Si = 3.52 mol
Mr " " = 28 g / mol
Mass = 3.52 mol * 28 g / mol
= 98.56 g
c. 0.550 mol of F₂
mol of F₂ = 0.55 mol
Mr " " = (19 * 2 ) g / mol
Mass = 0.55 mol * 38 g / mol
= 20.9 g
Hope I as clear in my answers and that they helped
