the approximate number of molecules in a grain of salt of size 1mm^3 is closest to what value?

Question

the options are 1.10^15 2. 10^17 3.10^21 4. 10^27 5. 10^33 .also plz explain the answer to me

Answer 1

the density of NaCl is 2.16 this means that 1cm^3 of NaCl weighs 2.16g
1mm^3 = 10^-3cm^3 and weighs 2.16*10^-3g
A mole of NaCl weighs 34.5+23=58.45g
58.45g =6.022*10^23 molecules
1molecule weighs 58.45 /6.022*10^23g =9.71*10^-23g
and so 2.16*10^-3/9.71*10^-23=2.25*10^19molecules