How many atoms does 0.5 kg sodium contain?

First calculate the moles of sodium

moles = 500 g/ 23 = 21.739 ...

Remember the Avogadro Number. 6.022 x 10^(23) is the number of atoms in one mole

Hence , multiplying

6.022 x 10^(23) X 21.739... = 1.309 x 10^(25) atoms in 0.5 kg of sodium .