Question

A gaseous hydrocarbon contains 82.76% of carbon. Given that it’s vapours density is 29, find it’s molecular formula.
[C = 12, H = 1]

August 5, 2024

Answer

Element	% composition	At. wt.	Relative no. of atoms	Simplest ratio
Carbon	82.76	12	82.76/12 = 6.89	6.89/6.89 = 1 × 2 = 2
Hydrogen	17.24	1	17.24/1 = 17.24	17.24/6.89 = 2.5 × 2 = 5

Simplest ratio of whole numbers = C : H = 1 : 2.5 = 2 : 5

Hence, the empirical formula is C₂H₅

Empirical formula weight = 2(12) + 5(1) = 29

V.D. = 29

Molecular weight = 2 × V.D. = 2 × 29 = 58

n = $Molecular\ weight\over Empirical\ formula\ weight$

= $58\over 29$ = 2

∴ Molecular formula = (C₂H₅)₂ = C₄H₁₀

💡 Some Related Questions

Copper [II] sulphate soln. reacts with sodium hydroxide soln. to form copper hydroxide according to the equation :
2NaOH + CuSO4 ⟶ Na2SO4 + Cu(OH)2↓
What mass of copper hydroxide is precipitated by using 200 g of sodium hydroxide.
[H = 1, O = 16, Na = 23, S = 32, Cu = 64]

Aluminium carbide reacts with water according to the following equation :
Al4C3 + 12H2O ⟶ 4Al(OH)3 + 3CH4
(i) State what mass of aluminium hydroxide is formed from 12 g. of aluminium carbide.
(ii) State the volume of methane at s.t.p. obtained from 12 g of aluminium carbide.
[relative molecular weight of Al4Cl3] = 144 ; Al(OH)3 = 78

How much calcium oxide is formed when 82 g. of calcium nitrate is heated. Also find the volume of nitrogen dioxide evolved :
2Ca(NO3)2 ⟶ 2CaO + 4NO2 + O2
[Ca = 40, N = 14, O = 16]

From the equation :
(NH₄)₂Cr₂O₇ ⟶ N₂(g) + 4H₂O(g) + Cr₂O₃
Calculate:
(i) the quantity in moles of (NH₄)₂Cr₂O₇ if 63 gm of
(NH₄)₂Cr₂O₇ is heated.
(ii) the quantity in moles of N₂ formed.
(iii) the volume in litres or dm3 of N₂ evolved at s.t.p.
(iv) the mass in grams of Cr₂O₃ formed at the same time.
[H = 1, Cr = 52, N = 14]

KClO3 ${\overset{ MnO2 }{\longrightarrow}}$ 2KCl + 3O2
(i) Calculate the mass of KClO3 required to produce 6.72 lit of O2 at s.t.p.
[K = 39, Cl = 35.5, O = 16]
(ii) Calculate the number of moles of O2 in the above volume and also the number of molecules.

P + 5HNO3 [conc.] ⟶ H3PO4 + H2O + 5NO2.
If 9.3 g of phosphorous was used in the reaction, calculate :
(i) Number of moles of phosphorous taken.
(ii) The mass of phosphoric acid formed.
(iii) The volume of NO2 produced at s.t.p.
[H = 1, N = 14, P = 31, O = 16]

Calculate the volume of oxygen required for the complete combustion of 8.8 g of propane [C3H8].
[C = 12, O = 16, H = 1, Molar Volume = 22.4 dm3 at s.t.p.]

Commercial NaOH weighing 30 g. has some NaCl in it. The mixture on dissolving in water and treatment with excess AgNO3 soln. formed a precipitate weighing 14.3 g. What is the percentage of NaCl in the commercial sample of NaOH.
NaCl + AgNO3 ⟶ AgCl + NaNO3
[Relative molecular mass of NaCl = 58 ; AgCl = 143]

From the equation : C + 2H2SO4 ⟶ CO2 + 2H2O + 2SO2
Calculate :
(i) The mass of carbon oxidized by 49 g of sulphuric acid [C = 12 ; relative molecular mass of H2SO4 = 98].
(ii) The volume of SO2 measured at s.t.p., liberated at the same time.

A sample of ammonium nitrate when heated yields 8.96 litres of steam (measured at step.)
NH4NO3 ⟶ N2O + 2H2O
(i) What volume of di nitrogen oxide is produced at the same time as 8.96 litres of steam.
(ii) What mass of ammonium nitrate should be heated to produce 8.96 litres of steam [Relative molecular mass of NH4NO3 is 80]