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Gas Laws

Skills to develop

  • Explain the:
  • Apply gas laws to solve problems involving gases.

    ABCD Laws of Gases

    The discovery of natural law is a scientific achievement. In terms of science, we are interested in the laws as well as the strategy leading to discovery. Science is the study of these laws and the development of scientific methods for discovery.

    The discoveries of gas laws represent some major breakthroughs in our understanding of the gases in the material world. At the dawn of science, experiments were performed on gases at specific conditions. Under these conditions, gas laws were formulated.

    A law is a model that mimics the behavior of a system. By applying a gas law, we can predict the outcome of certain parameters when a set of condition is understood or given.

    The ABCD laws of gases refers to four gas laws:

    Avogadro's law
    Boyles law
    Charles law
    Dalton's law of partial pressure

    These laws were not discovered in the above order, but we review them in this order. Since you also have some concept of the ideal gas law, we show how they are related to the ideal gas law,

    P V = n R T, where P, V, T, and n are pressure, volume, temperature, and amount of gas with R as the gas constant.

    The ideal gas law is easy to remember and apply in solving problems, as long as you get the proper values and units for the gas constant, R.

    The Avogadro's Law

    Equal volumes of gases have equal numbers of molecules at the same temperature and pressure. This was Avogadro's hypothesis in order to explain the simple ratios of volumes when gases react with one another. Now, we accept it as a law, because experiment shows that it is always true.

    Of course, this law can be and has been stated in many ways. Equal numbers of molecules means equal amount in moles.

    The Avogadro's law is part of the ideal gas law, P V = n R T, which can be written in the following form:

          R T
     V = ---- n
           P
    
    At some specific T and P, the volume is proportional to the amount, n, in moles (a fun link). Amounts of two gases at the same T and P are, of course, proportional to their volumes. Thus, when 2 L of hydrogen reacts with 1 L of O2, the number of hydrogen molecules are twice those of oxygen molecules. Observations like these led Avogadro to propose the diatomic molecules for these elements, and formulate the chemical reaction as: 2 H2 + O2 = 2 H2O The proposal was, and still is, a brilliant scientific reasoning.

    The Boyles Law

    At constant temperature, the pressure is inversely proportional to the volume of a definite amount of gas. This is known as the Boyles law.

    Robert Boyles (1627-1691), experimented with gas at constant temperature. Using Torricelli discovery, Boyle measured the variation of pressure when the volume changes, and discovered that volume is inversely proportional to the pressure, and vice versa.

    V = kb / P where kb is a constant. The Boyles law is also part of the ideal gas law, which can be written in the form: V = n R T / P At constant temperature, T, this formulation shows that V is inversely proportional to P.

    The Charles Law

    Jacques Charles (1746-1823) experimented with gas under constant pressure. In today's language, his discovery is that the volume of a gas is proportional to the temperature in K (kelvin). V = kc T, where kc is a constant. You should note that the absolute temperature scale (K) must be used for the above formula to be valid.

    Again, the Charles's law is also part of the ideal gas law, and the relationship between V and T is obvious:

    V = {n R / P} T, which is the same as the previous one if you assume kc = n R / P

    By now, you can see that the ideal gas law combines the ABC laws of gases.

    Well, there are four quantities in dealing with gases, amount n, volume V, pressure P, and temperature T. The ABC laws of gases give the relationship of any two of these quantities when the other two of them are held constant.

    Dalton's Law of Partial Pressure

    John Dalton (1766-1844) was the first to discover that in a container containing a mixture of gas, the total pressure is the sum of all partial pressures of its components.

    The partial pressure is the pressure due to a particular gas as if it is in the container by itself. Avogadro's law implies that gas molecule of any gas behave exactly the same way. Thus, the pressure exerted by n mole of any gas, or n mole of a gas mixture has the same pressure.

    The ideal gas law includes the law of partial pressures, because the total number of moles is the sum of moles of all the components in the mixture.

    ntotal = n1 + n2 + n3 + . . . + nn Since n = (V/RT) P, ni = (V/RT) Pi, and ntotal = (V/RT) Ptotal. Therefore, Ptotal = P1 + P2 + P3 + . . . + Pn As an example, the pressures of a 1-L container containing 0.10 mol N2 and 0.20 mol O2 mixtrue are:
    PN2 = 249.4 Pa.
    PO2 = 498.8 Pa.
    Ptotal = 748.2 Pa.

    On the Application of Gas Laws

    The Boyles law, Chales law, Daltons law and Avogadros law were discovered at the time when atomic theory and molecular theory began to develop. At that time, only macroscopic properties of gases were measured and these laws were discovered from imprecise measurements.

    Later, these laws are integrated into a simple ideal gas law for the calculation of gas properties. At temperatures much higher than the critical temperature of the gas, and when the pressure is not very high, the ideal gas law is adquate to predict the gas properties. However, for industrial and other applications accurate predictions are required, and corrections due to non-ideal behavior must be made.

    One of the applicatons of Dalton's law is for the correction of pressure when a gas is collected by displacement of water. During this process, the gas collected is saturated by water vapor. Water vapor pressure or partial pressure depends on temperature.

    Example

    A common method for preparing oxygen is the decomposition of KClO3 according to the reaction: 2 KClO3 = 2 KCl + 3 O2(g). The collection of O2 gas is usually by displacement of water. In an experiment, 0.250 L of O2 was collected over water, when the atmosphere pressure is 759 torr, and the temperature of water and the gas is 14 °C. Calculate the amount of O2 collected in moles. The vapor pressure of H2O at 14 °C is 12.0 torr.

    Solution

    The partial pressure of O2 is total pressure minus the partial pressure of water vapor.

    PO2 = (759 - 12) torr = 747 torr.

    By the ideal gas law, n = P V / R T, we have,

          (747/760 atm) * 0.250  L
    n = ----------------------------
         0.08205 L atm/K.mol * 287 K
    
      = 0.0104 mol.
    
    Discussion

    Note that 1 atm = 760 torr (or mm Hg). The units used in the formulation requires R = 0.08205 L atm / (mol.K).

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