Final answer:
To calculate the moles of magnesium, divide the mass of magnesium by the molar mass of magnesium.
Step-by-step explanation:
To calculate the moles of magnesium, we need to use the molar mass of magnesium (24.305 g/mol) and the given mass of magnesium (20g).
Moles of magnesium = mass of magnesium / molar mass of magnesium
Moles of magnesium = 20g / 24.305 g/mol
Moles of magnesium = 0.82 moles