Question

How many atoms in 12.5 grams of sodium?

Answer 1

22.99 mol in one gram of sodium, so: 12.5/22.99=0.543714659 mol.
Round it to whatever you need to. 

Answer 2

mol of Na =  $\frac{Mass of Na}{Molar Mass of Na}$
 
                =  $\frac{12.5g}{23.00g/mol}$
 
                =  0.54 mol

# atoms of Na =  mol ×  avogadro's constant
  
                       =  0.54 mol  ×  [6.02 × 10²³]
 
                       =  3.25 × 10²³