Question

carbon and oxygen combine in the ratio 3:8 by mass to form carbon-dioxide,deduce the ratio by numberof atoms in the compound

Answer 1

In carbon dioxide C:O = 3:8 (by mass)
Amount of C = 3x g
Amount of O = 8x g
We know:
Atomic weight of C = 12
Atomic weight of O = 16

So
Amount of C = 3x/12 gram-atom
Amount of O = 8x/16 gram-atom
1 gram-atom contains 6.022X10²³ number of atom

So
Number of C = (3x/12)X6.022X10²³
Number of O = (8x/16)X6.022X10²³

So number ratio of carbon and oxygen is (3x/12)X6.022X10²³ : (8x/16)X6.022X10²³
= 1:2