Réponse :
M(C12H22O11) = 12 * M(C) + 22 * M(H) + 11 * M(O) = 342
Or M(O) = 16 g.mol-1
Donc 12 * M(C) + 22 * M(H) + 11 * 16 = 342
Donc 12 * M(C) + 22 * M(H) + 176 = 342
Donc 12 * M(C) + 22 * M(H) = 342 – 176 = 166
Donc 6 * M(C) + 11 * M(H) = 166/2 = 83
Donc M(C) = (83 – 11 M(H)) /6
M(C4H7N3O) = 4 * M(C) + 7 * M(H) + 3 * M(N) + M(O) = 113
Or M(N) = 14 g.mol-1
113 = 4 [(83 – 11M(H))/6] + 7 * M(H) + 3 * 14 + 16
113 = 2/3 * 83 – 2/3 * 11 M(H) + 7 * M(H) + 42 + 16
113 = 166/3 – 22/3 M(H) + 21/ 3 * M(H) + 58
113 – 58 – 166/3 = -1/3 M(H)
55– 166/3 = -1/3 M(H)
(165 -166)/ 3 = -1/3 M(H)
-1/3 = -1/3 M(H)
M(H) = 1 g.mol-1
M(C) = (83 – 11 M(H)) /6
M(C) = (83 – 11 * 1) / 6 = 72/6 = 12 g.mol-1
M(C) = 12 g.mol-1
