[F.4 chem] mole ratio

Let CnHm be the formula of the straight-chain alkane.

C12H26 is cracked into C2H4 and CnHm­ in mole ratio 2 : 1. The equation is thus:
C12H26 → 2C2H4 + CnHm

No. of C atoms in the reactant = No. of C atoms in the products
12 = 2x2 + n
n = 8

No. of H atoms in the reactant = No. of H atoms in the products
26 = 2x4 + m
m = 18

Hence, the molecular formula of the straight-chain alkane = C­8H18

First, write the eqution:

我地let個straight-chain alkane做CxHy先:

由於the mole ratio of ethene: straight-chain alkane is 2 : 1, 所以"2C2H4 + CxHy"

C12H26 --> 2C2H4 + CxHy

x:
12 - 2(2) = 8
x=8

y:
26 - 2(4) = 18
y=18

所以molecular foumula of the straight-chain alkane 係 C8H18.