which topics I have to do for getting 50/70 in board cls 12 chem

The important topics of class 12 chemistry board exam are :

1. Solid State - Packing
2. Solutions - All the Colligative Props
3. Electro chemistry - Nernst eqn and all the formulas
4. Chemical Kinetics - Corresponding equations of all the orders and Activation Energy
5. Surface Chemistry - All definitions; and Colloids
6. Metallurgy - Remember all the ores of all metals and extraction processes
7. p block - trends in each group, uses of all compounds and reactions
8. d and f block - same as above
9. Coordination Chemistry - All definitions; CFT; Isomerism
10. RX ArX - All mechanisms (SN1 ,2, SE, NA)
11. ROH,ArOH,ROR - Rxns( Kolbe, Reimer-Teimann , Williamson synth) and mechanisms
12. RCHO, RCOR,RCOOH - All types of synthesis rxns and rxn of NH3 derivatives with carbonyl group
13. Amines - Hoffmann Bromamide, Gabriel Phthalamide, Carbylamine and Diazonium grp
14. Biomolecules - All rxns of Glucose, Hydrolysis products of polysaccharides and all protein structures
15. Polymers - Uses and monomers of all polymers, reactions
16. Chemistry In Every Day Life - Definitions of all medicinal compounds and atleast 2 eg of each; Soaps

Once you have done the topic please do see previous year papers of CBSE so that you get to know exactly what type of questions and what percentage of marks its constitute.

All the best for your Exams!