according to law of constant proportion --atoms combine in a fixed ratio by mass,what about in case of isotope

Question

for eg. co2---would combine in 3:8 ration but a carbon-14 would combine in 7:16 ratio

Answer 1

Yes, it would.

C + O2 --> CO2

The atomic mass of C = 12.0107 g/mole = 12.0
The atomic mass of O = 15.9994 g/mole = 16.0

12.0 : 16.0 *2 = 12.0 : 32:0 = 3:8

For Carbon 14:
The atomic mass of C = 14.0 g/mole
The atomic mass of O = 16.0 g/mole

14.0 : 16.0 *2 = 7:16