In $100$ $g$ $CaCO _3$, $40$ $g$ $Ca$ is present.

Calcium carbonate from any source contains 40% calcium. 

Law of Constant proportion states that a chemical compound always contains same elements combined together in the same proportion by mass.

LAW OF CONSTANT OR DEFINITE PROPORTION : It was given by Proust.
It states that all chemical compounds are found to have constant composition irrespective of their method of preparation or sources.
THE LAW OF MULTIPLE PROPORTION : It was given by Dalton.
It states that when one element combines with the other element to form two or more different compounds, the mass of one element which combines with a constant mass of the other bear a simple ratio to one another.

Law of constant composition is also called as the law of Definite proportion which says that the law of constant composition is a chemistry law which states samples of a pure compound always contain the same elements in the same mass proportion.

When hydrogen and oxygen combine to form water, they form water an extremely stable compound. We can say water is a stable compound because tremendous energy is released is a signal that hydrogen and oxygen are very eager to become water or in another way it is very hard to break water molecules apart into its components.

Law of definite proportion states that elements always combine in the definite ratio by mass. So it is in agreement with the law of definite proportion.

Nitrogen forms multiple compounds with oxygen. Hence, we cannot apply the law of definite proportion to nitrogen oxide.

Formula of Nitric oxide is NO. Mass of nitrogen in NO is 14 g. Mass of oxygen is 16. Therefore ratio is $14:16$ i.e $7:8$.

In a compound the ratio of the atoms or element by mass remains always same irrespective of the source of compound. This means a certain compound always formed by the combination of atoms in same ratio by mass. Hence, copper oxide made from any method will always give the same ratio of copper oxide.

Compounds are formed by the combination of two or more elements. In a compound the ratio of the atoms or element by mass remains always same irrespective of the source of compound. This means a certain compound always formed by the combination of atoms in same ratio by mass. If the ratio of mass of constituent atoms will be altered the new compound is formed.

In a compound the ratio of the atoms or element by mass remains always same irrespective of the source of compound. This means a certain compound always formed by the combination of atoms in same ratio by mass. If the ratio of mass of constituent atoms will be altered the new compound is formed.

Law of Constant Proportion states that a chemical compound always contains exactly the same proportion of elements by mass. This law is also known as Law of definite proportions. Joseph Louis Proust gave this law hence, this law is also known as Prousts Law.

Hydrogen combines with oxygen to form water. Water exists in solid ( ice), liquid ( water) and gas ( water vapor).

This reaction occurs in Haber's process-

In the first sample of copper oxide (obtained by the action of nitric acid on Cu), the mass of Cu is 2.16 g and the mass of oxygen is $\displaystyle 2.7 - 2.16  =  0.54$ g. 

First case :

In $100 $ g $\displaystyle CaCO _{3}$, weight of $Ca$ is $40 $ g.

law of multiple proportion is shown as copper reacts with two different compounds .

The percentage of copper and oxygen in a sample of CuO obtained from different methods were found to be same. This proves the law of Constant proportion as the ratio of Cu:O remains constant

Law of constant composition states that all samples of a given chemical compound have the same elemental composition by mass. 

Carbon combines with oxygen in $1:1$ ratio to form carbon monoxide and $1:2$ ratio to form carbon dioxide.

This law states that all samples of a given chemical compound have the same elemental composition by mass e.g. oxygen makes 8/9th of the mass of pure water while hydrogen makes up the remaining 1/9th of the mass.

Atoms of different elements combine in simple whole number ratios to form chemical compounds. Combination always takes place in the simplest possible way between particles of different weights.

The law of definite proportions and constant composition states that all samples of a given chemical compound have the same elemental composition by mass e.g. oxygen makes up up about 8/9th of mass of any sample of pure water while hydrogen makes up the remaining 1/9th of mass.

The law of multiple proportions state that if two elements form more than one compound between them then the ratios of the masses of the second element which combine with a fixed mass of the first element will be ratios of small whole numbers.

This is governed by the law of definite proportion. Carbon and oxygen combine in $3:8$ ratio. So $11 \ g$ of carbon dioxide is formed. 

First experiment
$\bullet$ Copper oxide $=1.375 g$
$\bullet$ Copper left $= 1.098 g$
$\bullet$ Oxygen present $= 1.375 - 1.098 = 0.277 g$
Percentage of oxygen in $CuO = (0.277) (100$ %) $1.375 = 20.15$ %

The law ofconstant proportion is given by Proust.

% of $H$$=$$\dfrac{12.4}{78}\times100$$=16$%

In chemistry, the law of definite proportions, sometimes called Proust's law or the law of definite composition, states that a chemical compound always contains exactly the same proportion of elements by mass and this law further says that, in any particular chemical compound, all samples of that compound will be made up of the same elements in the same proportion or ratio. For example, any water molecule is always made up of two hydrogen atoms and one oxygen atom in a ratio.

All of the following statements are correct for the composition of water.
A
For water, the gravimetric or mass ratio of hydrogen to oxygen is 1:8.
B
In water, the ratio of hydrogen and oxygen is 2:1 by volume.
C
The ratio in which hydrogen and oxygen combine gives the composition of water.
Note:
$2H _2 + O _2 \rightarrow 2H _2O$
Thus, the composition of water is $H : O = 2: 1 $
It is the ratio in which hydrogen and oxygen combine. The atomic masses of H and O are 1 and 16 respectively. Hence, for water, the gravimetric or mass ratio of hydrogen to oxygen is 2:16 or 1:8.
In water, the ratio of hydrogen and oxygen is 2:1
by volume. It is equal to the ratio in which hydrogen and oxygen combine.

$2 N _2 + 3 O _2 \rightarrow 2N _2 O _3$

Since, mass percentage of Ca in $CaCO _3 = 40 \text{%} $ and law of constant proportion is true.
So, the mass percentage of $Ca $ in $ 5gm$ $CaCO _3$ will also be $40\text%.$
Hence,$ 40\text{% of 5gm =}$$ 5\times $$\dfrac{40}{100}$ $= 2gm $
Hence, answer is option A.

Proust defined law of constant proportions. It is also called as $\text{Proust's law}$. it states that a given chemical compound always contains its component elements in fixed ratio by mass and does not depend on the method of preparation of the compound. 
Hence, answer is option D.

$ZnSO _4-22.65\% Zn $ $43.9\% H _2O$

$3H _2+N _2\rightarrow 3NH _3$
So 3 moles of hydrogen produced 1 mole of ammonia
1 mole hydrogen produces $X$ moles of ammonia
$X=\dfrac{1\times 1}{3}=\dfrac{1}{3}$ moles
Hence option $C$ is correct.

3 moles of hydrogen and 1 mole of nitrogen is convened completely into ammonia.

$3H _2+N _2\rightarrow 2NH _3$

Hence option C is correct.

So 4 volumes of reactants (3+1) and 2 volumes of products that is the ratio of initial and final is 2:1

Water will start comparing electrostatically,

According to the law of constant composition in a given compound, the elements always combine in the same proportion as each other.

$2CO + O _2 \rightarrow 2CO _2$

(A) A sample of $CaCO _3$ contains $Ca = 40\%$, $C = 12\%$ and $O= 48\%$. If the law of constant composition is true, then the mass of $Ca$ in $10$ g of $CaCO _3$ from another source is $\dfrac {40 \times 10 }{100}= 4.0 $ g

$14$ g of an element X combines with $16$ g of oxygen, then the element X could have an atomic weight of $7$ amu and its oxide is $X _2O $.
When the atomic weight is $7$, $14$ g will corresponds to $2$ moles.
$16$ g of oxygen corresponds to $1$ mole. 

$2KClO _3\rightarrow 2KCl+3O _2$
$2\times 122.5$ grams shows a wieght loss of  $3\times32 grams$
So 245 grams of $KClO _3$ is 100%
96 grams of $O _2$ is X%
$x=\frac{100\times 96}{245}=39.17$.
Hence option C is correct.

$Ag _2CO _3\rightarrow Ag _2O+CO _2(g)$
I mole of $Ag _2CO _3$ (275.74) grams  on decomposition gives a mass residue of 231.73 grams of $Ag _2O$ remaning mass is liberated as $CO _2$ gas.
If 1.38 grams $Ag _2CO _3$ is hetaed it produces X grams of mass residue of  $Ag _2O$
$X=\frac{1.38\times 231.739}{275.745}=1.16$ grams
Hence option A is correct.

$2H _2S+SO2\rightarrow 2H _2O+3S$

According to the equation $2\times 22.4 $ litre of  $H _2S$ gas reacts with $22.4$ litre of $SO _2$
So $5$ litre of $H _2S$ reacts with $10$ litre of $SO _2$
Hence option $A$ is correct.

The water vapor rises in the atmosphere due to heating of water by sunlight and there it cools down and forms tiny water droplets through something called condensation. When they all combine together, they grow bigger and are too heavy to stay up there in the air. This is when they will fall to the ground as rain, or maybe snow or hail by gravity.

The thermal resistance of heat transfer is low in drop wise condensation.

The evaporation takes place at all the time

Condensation is the process by which water vapor in the air is changed into liquid water.Cloud formation is caused by condensation of water vapor in the atmosphere condensing into a liquid state. Thus Option C is correct.

The process of changing water vapour into water is called condensation.

Evaporation causes cooling.

Evaporation is the process of a liquid converts into gaseous phase. In all these process liquid moles are evaporated into gaseous phase. Hence, option D is correct.

Wet clothes dry in sun due to evaporation the water in wet clothes takes heat energy from sun and gets evaporated.This process involves evaporation.
Hence, option A is correct.

Drying clothes is an example of evaporation as in evaporation water evaporates by the heat of the sun thus helping in drying of clothes .

Evaporation of the substance is the process in which the substance transition from liquid phase to gas phase. Condensation is the process in which gas returns to liquid form. Hence, options A is correct.

The fog appeared on sun glasses when gets in front of hot steam due to condensation.Condensation is the process where the vapor converts into liquid.
Hence option A is correct.

Evaporation is the process of a liquid converts into gaseous phase.In all these process liquid moles are evaporated into gaseous phase.So all the above processes involves evoporation.
Hence option D is correct.

The formation of water droplets on cold soda can while having soda in summer season is due to the process of condensation.

Condensation is the process where a gaseous substance converts into liquid.
Ex: Rainfall.