CnH2n+2 + Br2-----> CnH2n+1Br + HBr
la 1mol de alcan masa creste cu (80gBr-1gH)=> 79g
14n+2g alcan...........79g
100.g.........................109,72g
7900= 1536n +219,4-----> n=5------------------> pentan
are 2 izomeri :2-metil butan si 2,2-dimetil propan