First year chemistry: Chapter 4 Gases Short answer questions

Question. Write brief answers for the following.

Q.1. What is Absolute Zero and Absolute scale of temperature?

Answer.

Absolute Zero

The temperature at which the given volume of a gas reduces to zero is called Absolute zero.

The temperature at which the motion of gas molecules ceases is called Absolute zero.

On Celsius scale absolute zero is equal to -273.15 ^oC

On Kelvin scale absolute zero is equal to 0 K.

Absolute Scale of Temperature

Absolute Scale of Temperature was developed by Lord Kelvin . On Kelvin scale,

-273.15 ^oC (0K) is starting point of the scale. It is related to Celsius scale as follows

K = ^oC +273

Q.2: What is plasma?

Answer. Plasma is composed of a mixture of un-ionized gas, free electrons and positively charged particles.

Plasma is the fourth state of matter.

It was identified by William Crooks in 1879.

He heated molecules gas which changed into atomic form and then to ionic form at higher temperature.

Molecular gas ----> Atomic gas---> ions + electrons

At a high temperature of 10⁴ to 10⁶ K (10 eV to 100 eV), electrons are removed from the atom to form ions

Q.3: What are the different units of gas Constant R?

Answer.

The value of gas constant R depends upon the units of pressure and volume of a gas. Following are the common units of values of R.

R = 62.4 dm³ mm of Hg mole^-1K^-1 (Pressure in unit of mmHg and Volume in unit of dm³)

R = 62.4 dm³torr mole^-1K^-1 (Pressure in unit of torr and Volume in unit of dm³ )

R = 62.4 cm³torr mole^-1K^-1 (Pressure in unit of torr and Volume in unit of cm³ )

R = 62.4 J mole^-1K^-1 (Pressure in unit of Nm^-2 and Volume in unit of m³ )

Q.4: Define pressure. How will you explain it in different units?

Answer

Definition: Force per unit area is called pressure.

Mathematically, P = F/A

Units of Pressure:

SI Units

In SI system, unit of pressure is Pascal (Pa)

One Pascal is the force of one newton (N) applied on an area of 1m².

Thus, 1Pa = 1 Nm^-2

CGS system

In CGS system, unit of pressure is barye (Ba)

Thus, 1Ba = 1 dyn.cm^-2

Other units

Other units commonly used are atmosphere, torr, mm of Hg, Pound per square inch(psi)

An atmosphere (atm) is defined as

One atm is the force exerted by 76 cm long column of mercury on an area of 1cm³ at 0^oC.

Thus, 1atm = 101325 Nm^-2

1atm = 101325 Pa

101.325 KPa

1atm = 14.7 Psi

1atm = 760 Torr

1atm = 760 mmHg

Q.5: Define Charles Law? How will you derive Absolute Zero with its help.

Answer.

At constant pressure, the volume of a fixed mass of gas is directly proportional to the absolute temperature.

Mathematically,

V ∝ T

V = kT

V/T = k

Where, V is volume, T is absolute temperature and k is proportionality constant,

Derivation of Absolute law,

Let Volume of a gas at 0^oC = Vo

According to quantitative definition of Charles Law

Volume of gas at 1^oC = Vo + Vo(1/273)

Volume of gas at - 1^oC = V_o + V_o(-1/273)

Volume of gas at - 273^oC = V_o + V_o(-273/273) = 0

Thus at temperature of 273^oC, at which the volume of gas becomes zero is called absolute zero.

Q.6: What is Avogadro’s law

Answer. This law states that

Equal volumes of all gases at same temperature and pressure contain equal number of particles (molecules).

Thus, volume of a gas is directly proportional to the number of molecules

Mathematically,

V ∝ n

Where n = number of molecules of gas

V = volume of gas

Significance

Avogadro’s law can be used to determine

Mass

Volume

Number of molecules of a gas

Q.7: What is the difference between an ideal gas and a real gas?

Answer.

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