A sample of ammonia has a mass of 56.6 g. How many molecules are in this sample?

The formula for ammonia is NH3. If you check a periodic table, you can calculate the mass of 1 mole of NH3.

N = 14.01
H = 1.008

So the molar mass (mass/1 mole)
= 14.01 + 3 * 1.008
= 17.034

So there’s 17.034 g NH3 / 1 mole NH3

We want to convert grams to molecules:
56.6g NH3 * (1mol NH3/17.034g NH3)
= 3.323 mol NH3

1 mole of anything has 6.022 x 10^23 of that item.
So 3.323 mol NH3 would have 3.323 x 6.022×10^23 molecules of NH3.
3.323 * 6.022 x 10^23 = 1.945 x 10^24 molecules NH3

Ammonia’s molecular formula = NH3
Average Atomic Mass of…
N=14
H=1

NH3 =
(14+1(3))=17

56.6/17=3.329mol

1mol = 6.022*10^23

3.329*(6.022*10^23)=
2.0047238*10^24 molecules

Since question has 3 significant figures,
The 56.6g sample of NH3 has 2.00*10^24molecules.