Calculate the number of moles of magnesium in 5.0 g of magnesium

Use this formula: n = m : MM. You can find the molar mass in the periodic table. Recall that MM is in the units g/mol and cancels out the grams of the mass, only leaving the mols!

n (Mg) = 5 : 24.305 = 0.19758 ... mol

Since you gave me the answer in 2 significant figures, the answer is also in 2s. f. n (Mg) = 0.20mol

Let me know if you're confused about anything.