1. 163.26 g/mol C11H17N

186.24 g/mol C6H7N

312.29 g/mol C3H2N
3 Answers

1) molecualr formula is C11H17N (c weighs 12 g/mol, H = 1g/mol and N = 14 g/mol) so 11 x 12 + 17 + 14 = 163
2) C12H14N2 (C6H7N = 93 g/mol) 186/93 is 2 so double it all
3) C18H12N6 (C3H2N = 52 and 312/52 = 6, so multiply everything by 6)
I have carrid our rounding here so have not used the exact atmoic masses of each element, just ot save my time looking them up
the secret is that the empirical formula is simply the ratio of the elements at their lowest common denominator. so if you know the molecular weight of the actual molecule and you work out the empirical weight simply divide them. It should always be a whole number. Then simply multiply every element by that number
hope this helps

formula weight for
1) C11H17N=11X12+17X1+1X14=163 divide gm mole By this gives ratio as=1 so formula is C11H17N
2)C6+H7N=6X12+1X7+1X14=83 teh ratio of formula weigh and gm mole is 2,so formula is C12H14N2
3)C3H2N=3X12+2X1+14=52 the ratio of formula weight and gm mole is 6 so formula will be C18H12N6

3)C12H8N4