Question

The mass of an empty flask is determined on a balance. It is then filled with N2 and its mass measured. The same flask is then evacuated of N2 and refilled with an unknown gas at the same temperature and pressure and its mass determined.

Mass of empty flask =    88.15g

Mass of flask and unknown gas = 91.30g

Mass of flask and N2 =   89.67ga.

a. What is the relative mass of 1 molecule of the unknown to 1 molecule of N2?

b. What is the molar mass of the unknown gas?

c. If the unknown gas has 2 carbons to 5 hydrogens, what is the empirical formula?

d. What is the molecular formula of the unknown?

Answer 1

Mass of N₂ = 89.67 – 88.15 g = 1.52 g

Mass of unknown gas = 91.30 – 88.15 g= 3.15 g

Number of moles of N₂ = mass of N₂ / Molar mass of N₂

= 1.52 g / 28.0134 g = 0.05426

If we assume that the gases are ideal, then the number of moles of the unknown gas is equal to the number of moles of N₂ at same temperature, pressure and volume.

a. Relative mass of one molecule of unknown gas to one molecule of N₂ is 3.15 g /1.52 g = 2.0724

b. Molar mass of the unknown gas

= mass of the gas / number of moles of the gas

                                    = 3.15 g / 0.05426 = 58.0538 g

c. If the unknown gas 2 carbons to 5 hydrogens then the empirical formula is C₂H₅ because the molar mass of the empirical formula is 29.0611 g that is, half of the molar mass.

d. From c, we can deduce that the molecular formula of the unknown gas is C₄H₁₀