Laws of Stoichiometry

1. Law of Conservation of Mass:

• • The total mass of the reactant reacted is equal to the total mass of the product produced.
  • Considered a general reaction:
  $A+B⟶C+D$
• Let us consider a gm of A, b gm of B to give c gm of the product of C, and d gm of D.
• Then, $(a+b)gm=(c+d)gm$

2. Law of Constant Composition:

• A compound always contains the same elements combined in definite proportion by weight.
• It means the weight ratio of elements obtained from different sources by different methods of preparation remains fixed.
• Considered a general reaction:

SAMPLE: I $A+B⟶AB$

Let a gm of A, and b gm of B combined to give AB in Sample I

SAMPLE: II $A+BC⟶AB+C$

Let c gm of A and d gm of B combine to give AB in Sample II

Then, According to this law, $a:b=c:d$

Exception:

1. Metal sulphides and metal oxides like $PbS,HgS,A{l}_2{O}_3$ and don't$ZnO$ doesn't follow this law. And these compounds are called non-stoichiometric or Berthollite Compounds.
2. In Compounds prepared by different isotopes of the combining elements, the weight ratio of elements is different.

• $\%$ composition of Metal=Mass of Metal/(Mass of metal oxide)$\ast$$100$ $\%$

3.Law of Multiple PROPERTIES

• Two different elements can combine between themselves to give two or more different products, the ratio of the weight of one of the elements which combine with a fixed weight of another element is always simple whole-number multiple ratios.
• Let a gm of $A$ and b gm of $B$ combines to give $AB$ in sample I and 'c' gm of $A$ and 'd' gm of $B$ to give $A_{2}B$ in sample II.

Sample: I $A+B⟶AB$

Sample: II $A+B⟶A_{2}B$

In $AB$:

'a' gm of $A$ combines with 'b' gm of $B$

$1$ gm of $A$ combines with $b/a$ gm of $B$

$100$gm of $A$ combines with $b/a\ast 100$ gm of $B$ $A$

In $A_{2}B$:

'c' gm of combines with 'd' gm of $B$

$1$ gm of $A$ combines with $d/c$ gm of $B$

$100$ gm of $A$ combines with $d/c\ast 100$ gm of $B$

According to this law,

$\frac{b}{a}\times 100:\frac{d}{c}\times 100$

-This law is obeyed by oxides of $S,P,Pb,N,$etc.

4.Law of Reciprocal proportions:

-This law states that” Two different elements can combine separately with a fixed weight of the third element to give two different products, the ratio of the weight of two different elements which has reacted with a fixed weight of the third element is either the same or multiple or reciprocal when these two different elements are either the same or multiple or reciprocal when these two different elements combine between themselves to give the third product.”

-It is a special case of the law of equivalence proportions.

5.Law of gaseous volumes:

-Whenever gaseous react they do so in volumes that bear a simple ratio to one another and the volumes of the products if these are also gases provided all the measurements are made under similar temperature and pressure conditions.

Atomic Mass\Atomic Weight:

-Atomic weight of element is relative weight of element expressed using $\frac{1}{12}th$ of$C-12$ atom as reference element.

-Average mass of an atom.

-Based on relative natural abundance of that element's isotopes.

Eg: $Chlorine$ $(75\%ofC{l}^{35}\&25\%ofC{l}^{37})$