Answer is: the mass of platinum is 1.405 grams.
N(Pt) = 4,34·10²¹.
n(Pt) = N(Pt) ÷ Na.
n(Pt) = 4,34·10²¹ ÷ 6,022·10²³ 1/mol.
n(Pt) = 0,0072 mol.
m(Pt) = n(Pt) · M(Pt).
m(Pt) = 0,0072 mol · 195 g/mol.
m(Pt) = 1,405 g.
n - amount of substance.
Na - Avogadro number.
