chemistry, mass calculation

Question

the radioactive isotope ^60Co can be detected at amount of 10^(-11) g. Calculate the approximate number of cobalt atoms in this mass.

Answer 1

A mole of any atom has about 6.022 x 10^23 atoms, so we need to know how many moles of Co-60 there are in 10^-11 grams.

I looked up the atomic mass of Co-60 and found it to be 59.934 g/mole. The number of moles then would be

10^-11 g x 1 mole / 59.934 g. = 1.669 x 10^-12 moles.

The number of atoms then would be 6.022 x 10^23 atoms/mole x 1.669 x 10^-12 moles = 1.01 x 10^12 atoms.