Find the number of neutrons in 7mg of C-14

At wt of C-14 = 14g/mole and in 1 mole C-14 there are 6.022 x 10²³ atoms.

 

7 mg = 0.007 g

So, No of moles of C-14 = 0.007g / 14 g/mole  = 0.0005 moles

 

So No of C-14 atoms in 0.0005 moles =  0.0005 x 6.022 x 10²³ atoms 

= 3.00 x 10²⁰

 

In 1 C-14 there are 8 neutrons, so total neutrons = 8 x 3.00 x 10²⁰ = 2.4 x 10²¹