Calculate the number of molecules contained in 3.4 mg of ammonia.

Given:
m (NH3) = 3.4 mg = 0.0034 g
NA = 6.02 * 10 ^ 23 mol − 1

Find:
N (NH3) -?

Solution:
1) Using the periodic system of D.I.Mendeleev, calculate the molar mass of ammonia:
M (NH3) = Mr (NH3) = Ar (N) * N (N) + Ar (H) * N (H) = 14 * 1 + 1 * 3 = 17 g / mol;
2) Calculate the amount of ammonia substance:
n (NH3) = m (NH3) / M (NH3) = 0.0034 / 17 = 0.0002 mol;
3) Calculate the number of ammonia molecules:
N (NH3) = n (NH3) * NA = 0.0002 * 6.02 * 10 ^ 23 = 1.204 * 10 ^ 20.

Answer: The number of NH3 molecules is 1.204 * 10 ^ 20.