• * Gas consists of particles of negligible volume.
• * Particles exert no attractive forces on each other.
• Particles are in continuous random motion.
• The collisions between particles are perfectly elastic → no kinetic energy is lost on collision.
• The average kinetic energy of the particles is directly proportional to the absolute temperature (in Kelvins) i.e. average KE α absolute temperature (K)
* Important criteria in the definition of an ideal gas.
Boyle’s Law
At constant temperature, the volume of a fixed mass of gas is inversely proportional to its pressure.
P α 1/VPiVi = PfVf at constant T
It is important to recognize the different graphs that can be drawn under the different conditions (constant P, V, T). Commonly seen in MCQs.
Charles’s Law
At constant pressure, the volume of a fixed mass of gas is directly proportional to its absolute temperature (expressed in Kelvins, K).
V α TVi / Ti = Vf / Tf at constant P
Alternatively,
At constant volume, the pressure of a fixed mass of gas is directly proportional to its absolute temperature (expressed in Kelvins, K).
P α TPi / Ti = Pf / Tf at constant VWhen doing calculations involving T, make sure that they are expressed in Kelvins!
Question
Why is it that for variables like P and V, it is not always necessary to convert them into SI units but for T, it is always a must?
Click here for Answer
To convert P and V into S.I. units, the values must be multiplied by some constant. When taking ratios, the constants will cancel out thus it is not always necessary to convert these variables into their S.I. units.
However, to convert T from oC to K, the value must be added by 273.15. When taking ratios, the constant 273.15 will not cancel out thus it is always necessary to express T in the S.I. unit - Kelvins.
However, to convert T from oC to K, the value must be added by 273.15. When taking ratios, the constant 273.15 will not cancel out thus it is always necessary to express T in the S.I. unit - Kelvins.
Avogadro’s Law
Under conditions of constant temperature and pressure, equal volumes of gas contain equal number of molecules.
V α n at constant T & PDalton’s Law
The total pressure exerted by a mixture of gases which do not react is equal to the sum of the partial pressures of the constituent gases at the same temperature.
PT = PA + PB + PC + …PT = (nA + nB + nC + …) RT/VPA = (nA/nT) x PT => PA = (VA/VT) x PTIdeal Gas Equation
PV = nRTP : pressure of gas in Nm-2 or Pa.
V : volume of gas in m3
T : absolute temperature in Kelvins
n : number of moles of gas
R : universal gas constant (8.314 J K-1 mol-1)
When number of moles of gas is unchanged,
Try to derive the following equations for Mr from the ideal gas equation:
