The answer is 0.500979 mol.
The molar mass is the mass of 1 mole of the substance.
If the molar mass of nh4no3 is 80.0432 g/mol, that means there are 80.0432 g of substance in 1 mole. The question is how many moles is 40.10 g of substance.
80.0432 g : 1 mol = 40.10 g : x mol
By crossing the products:
x · 80.0432 g = 1 mol · 40.10 g
⇒ x = 40.10 ÷ 80.0432 mol = 0.500979 mol.
