What is the mass of a sample of NH3 containing 6.3 * 1024 molecules of NH3?

157 grams

178 grams

182 grams

198 grams

178 grams

Explanation:

It is known that 1.0 mole of a compound contains Avogadro's number of molecules (6.022 x 10²³).

Using cross multiplication:

1.0 mol contains → 6.022 x 10²³ molecules.

? mol contains → 6.3 x 10²⁴ molecules.

∴ The no. of moles of (6.3 x 10²⁴ molecules) of NH₃ = (1.0 mol) (6.3 x 10²⁴ molecules) / (6.022 x 10²³ molecules) = 10.46 mol.

∴ The no. of grams of NH₃ present = no. of moles x molar mass = (10.46 mol) (17.0 g/mol) = 177.8 g ≅ 178 g.