Bonjour,

1) n(A) = m(A)/M(C₃H₈)

m(A) = 1,41 g et M(C₃H₈) = 3xM(C) + 8xM(H) = 3x12,0 + 8x1,0 = 44,0 g.mol⁻¹

⇒ n(A) = 1,41/44,0 ≈ 3,20.10⁻² mol

2) Vm(A) = V(A)/n(A) = 0,80/3,20.10⁻² ≈ 25 L.mol⁻¹.

3) V(B) = 2 x V(A) = 2 x 0,80 = 1,60 L

Mêmes conditions de température et de pression

⇒ Vm(B) = Vm(A) = 25 L.mol⁻¹

⇒ n(B) = V(B)/Vm(B) = 1,60/25 = 6,4.10⁻² mol

⇒ M(B) = m(B)/n(B) = 3,71/6,4.10⁻² ≈ 58 g.mol⁻¹

4) M(CₓH₂ₓ₊₂) = x.M(C) + (2x + 2).M(H)

= 12x + (2x + 2)

= 14x + 2

⇒ 14x + 2 = 58 ⇔ x = 56/14 = 4

⇒ L'alcane inconnu est du C₄H₁₀ : Butane