N = n x No
No = avogadro's number = 6.02 x 10²³
n = N / No
n = 15 / 6.02 x 10²³
n = 2.49 x 10⁻²³ moles
the mass of one atom:
2.09 x 10⁻²¹ : 15 = 1.393 x 10⁻²² g
molar mass(MM) of the element:
MM = mass : mole
MM = 1.393 x 10⁻²² : 2.49 x 10⁻²³
MM = 5.594 = 6
the closest is Lithium
Mass for 5 moles:
mass = mole x molar mass
mass = 5 x 5.594
mass = 27.97 g