Bonjour,
Exercice 1
1) M(CuSO₄,5H₂O)
= M(Cu) + M(S) + 4M(O) + 10M(H) + 5M(O)
= 63,5 + 32 + 4x16 + 10x1 + 5x16
= 249,5 g.mol⁻¹
2) m = 20 g
n = m/M = 20/249,5 ≈ 8,0.10⁻² mol
3) n = 0,16 mol
m = n x M = 0,16 x 249,5 ≈ 40 g
Exercice 2
c = 0,25 mol.L⁻¹
1) M(C₁₂H₂₂O₁₁) = 12xM(C) + 22xM(H) + 11xM(O)
= 12x12 + 22x1 + 11x16
= 342 g.mol⁻¹
2) Pour préparer 1 L de solution de concentration c, il faut prélever :
n = c x V = 0,25 x 1 = 0,25 mol
3) m = n x M = 0,25 x 342 = 85,5 g
4) Cf = Cm/10 = 0,25/10 = 0,025 mol.L⁻¹
Exercice 3
1) m = V x ρ = 50 x 700 = 35 000 g = 35 kg
2) et 3) 2C₈H₁₈ + 25O₂ → 16CO₂ + 18H₂O
4) M(C₈H₁₈) = 8xM(C) + 18xM(H) = 8x12 + 18x1 = 114 g.mol⁻¹
5) n = m/M = 35000/114 ≈ 307 mol
