Calculate the number of molecules in 42.5 g NH3.

To calculate the number of molecules in 42.5 g of NH3, we first need to determine the molar mass of NH3.

The molar mass of NH3 is:
N = 14.01 g/mol
H = 1.01 g/mol (3 hydrogen atoms in NH3)

Molar mass of NH3 = 14.01 + (1.01 x 3) = 17.03 g/mol

Next, we need to convert the given mass of NH3 (42.5 g) to moles using the molar mass:

42.5 g NH3 x (1 mol NH3 / 17.03 g NH3) = 2.50 mol NH3

Now, we can use Avogadro's number to calculate the number of molecules in 2.50 moles of NH3:

2.50 mol NH3 x 6.022 x 10^23 molecules/mol = 1.51 x 10^24 molecules

Therefore, there are approximately 1.51 x 10^24 molecules in 42.5 g of NH3.