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Q.
The emperical formula of a compound is $C _{2} H _{5} O$ and its vapor density is $45$ . What is the molecular formula of the compound?
TS EAMCET 2020
Solution:
Given, empirical formula of compound $= C _{2} H _{5} O$
Vapour density $=45$
Molecular formula $=?$
Empirical formulas show the simplest whole number ratio of atoms in a compound.
Molecular formula show the number of each type of atom in a molecule.
Relation between empirical formula and molecular formula.
Molecular formula $=n$ (empirical formula)
$n=\frac{ MM \text { (molar mass) }}{ EFM \text { (empirical formula molar mass) }}$
Molecular weight $=2 \times$ vapour density $=2 \times 45=90$
Let, the molecular formula be $C_{2 x} H_{5 x} O_{x}$.
Therefore,
$n=\frac{90(\text { molecular weight })}{47(E F M)}=1.47 \approx 2$
Value of $x=2$
Molecular formula
$=\left( C _{2} H _{5} O \right)_{2}= C _{4} H _{10} O _{2}$