Name: Law of Multiple Proportions
Uploaded: 2019-07-19
Duration: 6 min 43 s
Description: The Law of multiple proportions is applicable only when two elements combine to form two or more compounds. Watch this video to know how this law is applicable.

Question

State and explain Avogadro's law. How does it explain Gay-Lussac's law of gaseous volumes. How it could discard Berzelius's hypothesis?

Accepted Answer

Avogadro's law states that equal volumes of all gases at the same temperature and pressure contain an equal number of molecules. Avogadro's hypothesis explains correctly the reaction between hydrogen and oxygen forming water vapour.

$2 H_{2} + O_{2} \to 2 H_{2} O 2 vol 1 vol 2 vol$

$2$ volumes of hydrogen gas combine with $1$ volume of oxygen gas to form $2$ volumes of water vapour measured under the same condition of temperature and pressure.

Explanation of Gay Lussac's law of gaseous volumes by Avogadro's hypothesis: In the chemical combination of gases $A$ and $B$ to form a gaseous compound $AB$ , Let $' a'$ molecules of gas $A$ combine with $' b'$ molecules of gas $B$ ( $a$ & $b$ are simple integers).

Under the same condition of temperature and pressure, if $1 cc$ of gas $A$ contains $n$ molecules then $1 cc$ of gas $B$ would also contain the same number of $n$ molecules. This is as per Avogadro's hypothesis.

Since $1 cc$ of the gas $A$ contains $n$ molecules then 'a' molecules of gas $A$ would be present in $(\frac{a}{n}) cc$ .

And similarly $' b'$ molecules of the gas $B$ would be present in $(\frac{b}{n}) cc$ .

Thus, the volumes of gases $A$ and $B$ will combine in the ratio $a : b$ (a simple ratio, since $a$ and $b$ are whole members), when the volumes are measured at the same temperature and pressure. Avogadro's hypothesis thus, explains the law of gaseous volumes.

Berzelius hypothesis states that equal volumes of all gases at the same temperature and pressure contain an equal number of atoms. This hypothesis when applied to some gaseous reactions failed to explain Dalton's atomic theory.

Example: It was observed experimentally that for a reaction,

$H_{2} + {Cl}_{2} \to 2 HCl 1 vol 1 vol 2 vol$

$1$ volume of hydrogen combines with $1$ volume of chlorine to produce $2$ volumes of hydrogen chloride, the volumes measured under similar conditions of temperature and pressure.
Let $1$ volume of a gas contain $' n'$ atoms and applying Berzelius hypothesis, $n$ atoms of hydrogen combine with $n$ atoms of chlorine to form $2 n$ atoms of hydrogen chloride.
Or, $1$ atom of hydrogen combines with $1$ atom of chlorine to form $2$ atoms of hydrogen chloride.
Or, $1$ atom of hydrogen chloride contains $\frac{1}{2}$ atom of hydrogen and $\frac{1}{2}$ atom of chlorine.

The above statement contradicts Dalton's atomic theory that says atoms are indivisible.

State and explain Avogadro's law. How does it explain Gay-Lussac's law of gaseous volumes. How it could discard Berzelius's hypothesis?

Important Questions on Basic Concepts Atoms & Molecules

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