Tag: law of definite proportions

Questions Related to law of definite proportions

64g of an organic compound has 24 g carbon and rest hydrogen and oxygen.  The empirical formula of the compound is

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  1. ${C _2}{H _8}{O _2}$

  2. ${C _2}{H _4}O$

  3. $C{H _4}O$

  4. $C{H _2}O$

Show answer
Correct Option: C

Zinc sulphate contains $22.65$% $Zn$ and $43.9$% ${H} _{2}O$. If the law of constant proportions is true, then the mass of zinc required to give $40g$ crystal will be:

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  1. $9.06g$

  2. $90.6g$

  3. $0.906g$

  4. $906g$

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Correct Option: A
Explanation:
100g of crystals are obtained from = 22.65g of $Zn$

$\therefore $ 40 g of crystals are obtained from =$\frac { 22.65 }{ 100 } \times40$

= 9.06g 

Which of the following statements is/are correct?

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  1. 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 $4.0$ g.

  2. $12$ g of carbon is heated in vacuum and there is no change in the mass. This is the best example of the law of conservation of mass.

  3. Air is heated at constant pressure and there is no change in mass but the volume increases. This is the best example of the law of conservation of mass.

  4. $SO _2$ gas was prepared by (i) heating $Cu $ with conc. $H _2SO _4$ (ii) burning sulphur in oxygen (iii) reacting sodium sulphite $(Na _2SO _3)$ with dilute $H _2SO _4$. It was observed that is each case, $S$ and $O$ combine in the ratio of $1:1$. This data illustrates the law of constant composition.

Show answer
Correct Option: A,D
Explanation:

(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


According to the law of constant composition, all samples of a given chemical compound have the same elemental composition by mass.
Hence, the statement A is true.

(B) $12$ g of carbon is heated in vacuum and there is no change in the mass. This is not the best example of the law of conservation of mass as there is no chemical transformation involved since there is a vacuum.
The Law of Conservation of Mass states that matter can be changed from one form into another, mixtures can be separated or made and pure substances can be decomposed but the total amount of mass remains constant.
Hence, the statement B is false.

(C) Air is heated at constant pressure and there is no change in mass but the volume increases. This is not the best example of the law of conservation of mass as there is no chemical transformation involved since there is a vacuum.

The Law of Conservation of Mass states that matter can be changed from one form into another, mixtures can be separated or made and pure substances can be decomposed but the total amount of mass remains constant.
Hence, the statement C is false.

(D) $SO _2$ gas was prepared by (i) heating $Cu$ with conc. $H _2SO _4$ (ii) burning sulphur in oxygen (iii) reacting sodium sulphite $(Na _2SO _3)$ with dilute $H _2SO _4$. It was observed that is each case, S and O combine in the ratio of $1:1$. This data illustrates the law of constant composition.

According to the law of constant composition, all samples of a given chemical compound have the same elemental composition by mass.

Hence, the statement D is true.

$14$ g of element X combines with $16$ g of oxygen. On the basis of this information, which of the following is a correct statement?

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  1. The element X could have an atomic weight of $7$ amu and its oxide is $XO$

  2. The element X could have an atomic weight of $14$ amu and its oxide is $\displaystyle X _{2}O$

  3. The element X could have an atomic weight of $7$ amu and its oxide is $\displaystyle X _{2}O$

  4. The element X could have an atomic weight of $14$ amu and its oxide is $\displaystyle XO _{2}$

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Correct Option: C
Explanation:

$14$ 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. 

The formula $X _2O $ suggests the valency of $1$ for X.

The % loss in mass after heating a pure sample of potassium chlorate (Mol. mass = 122.5) will be:

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  1. 12.25

  2. 24.50

  3. 39.17

  4. 49.0

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Correct Option: C
Explanation:

$2KClO _3\rightarrow 2KCl+3O _2$
$2\times 122.5$ 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.

The volume of oxygen at NTP evolved when 1.70 g of sodium nitrate is heated to a constant mass is: 

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  1. 0.112 litre

  2. 0224 litre

  3. 22.4 litre

  4. 11.2 litre

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Correct Option: B
Explanation:
The chemical equation is:

$2NaNO _3(s)→  2NaNO _2 (s)+ O _2(g)$

2 moles of NaNO$ _3$ give one mole of Oxygen/22.4 L oxygen at STP

weight of NaNO$ _3$ = 85g

2 moles of NaNO$ _3$ weigh = 2(23+14+48) = 170g

170 g of NaNO$ _3$ will produce 22.4 L Oxygen at STP

​1.70 g will produce $= \cfrac{1.70}{170}×22.4 =0.224$ litres

Hence, the correct option is $\text{B}$

The mass of residue left after strongly heating 1.38 g of silver carbonate will be: 

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  1. 1.16 g

  2. 1.33 g

  3. 2.66 g

  4. 1.08 g

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Correct Option: A
Explanation:

$Ag _2CO _3\rightarrow Ag _2O+CO _2(g)$
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.

At NTP, 10 litre of hydrogen sulphide gas reacted with 10 litre of sulphur dioxide gas. The volume of gas, after the reaction is complete, would be:

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  1. 5 litre

  2. 10 litre

  3. 15 litre

  4. 20 litre

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Correct Option: A
Explanation:

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

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.

Which of the following is a limitation of the law of constant composition is not applicable?

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  1. When same isotopes of an element take part in the formation of a particular compounds.

  2. When different isotopes of different elements take part in the formation of a particular compound.

  3. When different isotopes of an element take part in the formation of a particular compound.

  4. None of the above.

Show answer
Correct Option: C
Explanation:
Law of constant composition is not applicable if an element exists in different isotopes which may be involved in the formation of the compound. The elements may combine in the same ratio but the compounds formed may be different.

Hence, the correct option is $(C)$
Assertion (A): Pure water obtained from different sources such as a river, well, spring, the sea always contains hydrogen and oxygen in the ratio of 1:8 by mass.

Reason (R): The total mass of reactants and products during chemical or physical change is always the same.
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  1. Both A and R are true and R is the correct explanation of A

  2. Both A and R are true and R is not correct explanation of A

  3. A is correct and R is incorrect

  4. A is incorrect and R is correct

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Correct Option: A