(i) Law of constant proportion states that, “a pure chemical compound always consists of the same elements that are combined together in a fixed (or definite) proportion by mass”.
(ii) Carbon : oxygen (by mass) = 3:8
i.e. 3 g of carbon requires 8 g of oxygen to form carbon dioxide.
.'. 9 g of carbon require (3 X 8) 24 g of oxygen to form carbon dioxide.