Laws of Stoichiometry
Stoichiometry:
Stoichiometry is the branch of chemistry which deals with the calculations involving mass and volume relations among reactants and products in a chemical reaction. (In Greek, Stoichiometry means to measure of an element).
The laws of stoichiometry are as follows:
i) Law of conservation of mass.
ii) Law of constant proportions/Law of definite proportions.
iii) Law of multiple proportions.
iv) law of reciprocal proportions.
v) Gay lussac's law of gaseous volume.
i) Laws of conservation mass:
It states that "The total mass of the reactants is equal to the total mass of the products in a chemical reaction". Or, "Mass is neither created nor destroyed in chemical reaction."This law is related with indestructibility of matter
For a general chemical reaction, A + B ---->C+D
Let W₁ gm of A reacts with W₂ gm of B to give W3 gm of C and W4 gm of D.
According to the law of conservation of mass, W₁+ W₂ = W3 + W4
This means that there is no loss in mass in a chemical reaction. That's why this law is also known as law of indestructibility of matter.
***State law of conservation of mass. Why is this law known as law of indestructibility of matter?
*** How does Daltons atomic theory explain the law of conservation of mass?Ans, According to law of conservation of mass." Mass can neither be created nor destroyed or total mass of the reactant's is equal to the total mass of the products during chemical reaction. According to Dalton's atomic theory, atoms neither can be created nor can be destroyed during a chemical reaction, simple rearrangement of atoms takes place. Since, an atom has a fixed mass, total mass during a reaction is conserved, which is same as the law of conservation of mass. Hence, Dalton's atomic theory explain the law of conservation of mass.
*** 12gms of carbon react with 32gms of oxygen to produce 44gms of CO2. Which chemical law do these data illustrate? State the law.
ii) Law of constant proportions or law of definite proportions:
It states that "The same chemical compound always contains the same elements combined together in definite proportions by weight regardless of the origin or mode of formation of the compound."
For eg, If pure water collected from different places (like Nepal, China, India,America etc) as well as different sources(like rain water, river, lake, sea water, tap watre, underground water etc,) are analysed, all water samples contain only two elements hydrogen and oxygen combined together in the ratio of 1:8 by weight.
*** 5.40 gm of copper oxide was obtained by treating 4.32 gm of copper with nitric acid and subsequent ignition .Similarly 2.30 gm of copper oxide on reduction gave 1.84 gm of copper in another experiment.Show that these data illustrate the law of constant composition.
iii) Law of multiple proportions:
It states that "When one element combines with another element to form two or more different compounds, then the weight of one of the element which combines with the constant weight of the other element bear a simple whole number ratio to one another."
For eg, hydrogen and oxygen combine with each other to form water and hydrogen peroxide.
In water: 2 parts by wt. of hydrogen combines with 16 parts by wt. of oxygen.
In hydrogen peroxide: 2 parts by wt. of hydrogen combines with 32 parts by wt. of oxygen.
Thus, the wt. of oxygen which combines with the same wt. of wt.) are in the rati hydrogen (2 parts by wt.) are in the ratio 16:32 or 1:2 which is a simple whole no. ratio and hence, it illustrates the law of multiple proportions.
*** phosphorous reacts with oxygen to produce P₂ 03 and P₂ 05 respectively. Which chemical law does the data illustrate? State the law.
*** H and O reacts separately to give H₂ O2 and H₂ O respectively. What law of stoichiometry is illustrated? State the law.
*** A certain element X, forms three different binary compounds with chlorine containing 50.68%,68.95% and 74.75% chlorine respectively. Show that how these data illustrate the Law of multiple proportions.
iv) Law of reciprocal proportions / law of equivalent proportions:
It states that "when two different elements combine separately with the fixed weight of third element, then the ratio of their weights in which they do so is either same or some simple multiple of the ratio in which they combine with each other.
Illustration: Consider three different elements like sulphur hydrogen and oxygen which combines to give H2S,H2O and SO2 .
In H2S: 2 parts by wt. of hydrogen combine with 32 parts by wt. of sulphur.
In H2O:2 parts by wt. of hydrogen combine with 16 parts by wt. of oxygen.
The weights of sulphur and oxygen which combine separately with the fixed wt. of hydrogen ( 2 parts by wt.) are in the ratio of 32:16 or 2:1.
i.e. Sulphur : Oxygen=2:1 ..............(1)
Now, both sulphur and oxygen combine with each other to form SO2.
In SO₂: The ratio of weights of sulphur and oxygen is 32:32 or 1:1 which is the simple multiple of the ratio 2:1 as in H2S and H₂O i.e. eqn(1).
Here, 1:1 is the simple number multiplication of 2:1 (i.e. 1/1×2=2/1). Hence, it illustrates the law of reciprocal proportions.
v) Gay Lussac's law of gaseous volume:
It states that "When two different gases combine together under similar conditions of temperature and pressure to give the gaseous products, the volumes of the gaseous reactants as well as the products bear a simple whole number ratio."
For eg, hydrogen and chlorine in their gaseous state react to give the gaseous product hydrogen chloride.
Hydrogen (g) + Chlorine(g)..............................>
1vol. 1vol.
Hydrogen chloride (g)
2vol.
It has been experimentally found that one volume of hydrogen combines with one volume of chlorine to give two volumes of hydrogen chloride gas. The ratio of volumes of hydrogen, chlorine as. The ratio of and hydrogen chloride is 1:1:2 which is simple whole number ratio and hence, it illustrates the law of gaseous volume.
*** One volume of hydrogen reacts with one volume chlorine to give two volumes of hydrogen chloride gas.Which law of stoichiometry is illustrates? State the law.