11th Class Chemistry Chapter three Short Questions with Answers Solved

Question: 1

What are isotherms? What happens to the positions of isotherms when they are plotted at high temperature for a particular gas?

Answer: 1

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Isotherms are the graphs between pressure and volume at constant temperature and number of moles. This condition is fulfilled by Boyle's law. The word isotherm means "same temperature".

Question: 2

The product of pressure and volume at constant. Temperature and number of moles is a constant quantity. Why?

Answer: 2

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When the temperature and number of moles of a gas are constant, then the increase of pressure decreases the volume in such a way that PV remains constant (PV = K), by doubling the pressure the volume becomes half. We can say that, P₁V₁ = P₂V₂ = P₃V₃, at constant temperature and number of moles.

Question: 3

Justify that 1 cm³ of H₂ 1 cm³ of CH₄ at STP will have same number of molecules. When one molecule of CH₄ is 8 times heavier that of hydrogen.

Answer: 3

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According to Avogadro’s law, equal volumes of the ideal gases at same temperature and pressure have equal number of molecules. So 1cm³ of H₂ and 1 cm³ of CH₄ at STP will have an equal number of molecules. No doubt, the molecule of methane is eight times heavier than H2, but the sizes of the gas molecules and their masses don't disturb the volumes. The reason is that at STP, one molecule of the gas is at a distance of three hundred times than its diameter.

Question: 4

Why is the Boyle's law applicable only to the ideal gases?

Answer: 4

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Boyle's law is applicable to those gases which have no forces of attractions among the molecules. Such gases are ideal. So, Boyle's law is applicable to only ideal gases.

Question: 5

What are various scales of thermometry?

Answer: 5

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Question: 6

What is absolute zero? What happens to real gases while approaching it?

Answer: 6

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It is the lowest possible temperature which would have been achieved if the substance remains in the gaseous state. All the real gases are converted to liquids above this temperature. This -273.16°C is called zero absolute or zero Kelvin.

Question: 7

How the density of an ideal gas doubles by doubling the pressure or decreasing the temperature on Kelvin scale by 1/2?

Answer: 7

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Question: 8

Why do we get a straight line when pressures are plotted against inverse of volumes? This straight line changes its positions in the graph by varying the temperature. Justify it.

Answer: 8

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When the pressure of a gas is plotted against 1/V, we get a straight line at constant temperature. The reason is that P and 1/V are directly proportional to each other, with power unity on both variables. When the temperature
changes then value of p changes for same 1/V value.

Question: 9

Justify that the volume of given mass of a gas becomes theoretically zero at -273^°C.

Answer: 9

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Question: 10

Calculate the S.I unit of R.

Answer: 10

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Question: 11

Charles's law is not obeyed when the temperature is measured on Celsius scale. Justify it.

Answer: 11

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Question: 12

How the value of the general gas constant 'R' can be derived with the help of Avogadro's law?

Answer: 12

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Question: 13

Gases deviate more from the general gas equation at 0° C and deviate to less extent at 100° C. why?

Answer: 13

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At 0° C, the forces of attractions are dominant and gases become non-ideal. At high temperature attractive forces become less dominant and gases behave ideally.

Question: 14

Dalton's law of partial pressures is only applicable to those mixtures of gases which are Ideal and they do not react with each other under the given conditions. Why?

Answer: 14

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Dalton's low is applicable to the mixture of gases. All the gases in the mixture could have no forces of attractions or repulsions, so that they may be able to exert their own pressures independently. This is only possible when the gases are non-polar.

Question: 15

Lighter gases diffuse more rapidly than heavier gases. Give reason.

Answer: 15

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At a given temperature the average. K.E. of different gas molecules are same. Since their masses are different, so their velocities will also be different. The lighter molecules will have greater velocities and so they WI diffuse rapidly.

Question: 16

How the behavior of real gases is given by van der waal's equation?

Answer: 16

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The constant 'a' and 'b' called van der Waal's constants give the quantitative measurements of attractive forces and sizes of the gas molecules. These parameters are very important for the real gases.

Question: 17

Why the gases deviate from ideal behavior at high pressure and low temperature?

Answer: 17

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When the temperature of the gases are low, the attractive forces become dominant, so gases don't obey the gas laws. When the pressure of the gases are high, collisions become more frequent and force of attraction are created. Moreover, the actual volume of the gas molecules are no more negligible as compared to the volume of the vessel.

Question: 18

What are application of plasma?

Answer: 18

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(i) Glowing plasma inside the bulb

(ii) Neon signs

(iii) Processing of semi-conductors

(iv) Sterilization of some medical products.

(v) Lamps

(vi) Lasers etc.

Question: 19

Why the non-polar gases like H₂ and He have a very low critical temperatures while polar gases like NH₃ and SO₄ have critical temperatures, sufficiently above room temperature?

Answer: 19

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H₃ and He are consisted of small-sized molecules, and have low polar abilities. They have least forces of attractions among themselves. In order to liquefy them, their temperatures have to bring close to absolute-zero. NH₃ and SO₂ have attractive force event at room temperature. In order to liquefy them their temperature may be above the room temperature.

Question: 20

Give two causes for deviation of gases from ideality?

Answer: 20

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The causes are due to two faulty assume`

Actual volume of the gas molecules is negligible as compared to the volume of vessel.

There are no forces of attraction among the molecules of gases.

These two postulates are correct when the temperature is high or pressure is low.

Question: 21

The amount of pressure which is decreased due to the forces of attraction is given by a/V₂ where 'a' is the van der Waal's constant and V is the volume of the vessel.

Answer: 21

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The pressure p is proportional to the number of molecules which are hitting on the walls of the vessel/area/sec. The number of molecules/area/sec. Is proportional to the density of the gas. P' (lessened pressure) depends upon the number of molecules which are attracting each other.

Question: 22

In Joule-Thomson effect sudden expansion of the gas molecules needs energy. Why?

Answer: 22

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In the compressed state, there are sufficient attractive forces among. The molecules of the gas. During sudden expansion, the energy is required to overcome the intermolecular attractions. Moreover, the molecules need extra energy to run away in vacuum.

Question: 23