Hey there!
Molar mass Na2SO4 = 142.04 g/mol
Number of moles:
n = m / mm
n = 25 / 142.04
n = 0.176 moles of Na2SO4
Therefore, use the Avogadro constant
1 mole Na2SO4 ------------------- 6.02x10²³ molecules
0.176 moles Na2SO4 ------------ molecules ??
0.176 x ( 6.02x10²³ ) / 1
=> 1.059x10²³ molecules of Na2SO4
hope this helps!