Combining volumes, Law

Combining volumes, law of, 26, 236 Combustion, heat of hydrogen, 40 Complex ions, 392 amphoteric, 396 bonding in, 395 formation, 413 geometry of. 393 in nature, 396 isomers, 394 linear, 395 octahedral, 393 significance of, 395 square planar, 395 tetrahedral, 394 weak acids, 396 Compound, 28 bonding in, 306 Concentration and equilibrium, 148 and E zero s, 213 and Le Chatelier s Principle, 149 effect on reaction rate, 126, 128 molar, 72... 

Perhaps the first stoichiometric relationship to be discovered was the law of combining volumes, proposed by Gay-Lussac in 1808 The volume ratio of any two gases in a reaction at constant temperature and pressure is the same as the reacting nude ratio. 

The law of combining volumes, like so many relationships involving gases, is readily explained by the ideal gas law. At constant temperature and pressure, volume is directly proportional to number of moles (V = kin). It follows that for gaseous species involved in reactions, the volume ratio must be the same as the mole ratio given by the coefficients of the balanced equation. 

Avogadro suggested this relationship to explain the law of combining volumes. Today it seems obvious. For example, in the reaction... 

According to the combined gas law, the volume of a given mass of gas can have any value, depending on its temperature and pressure. To compare the quantities of gas present in two different samples, it is useful to adopt a set of standard conditions of temperature and pressure. By universal agreement, the standard temperature is chosen as 273 K (0°C) and the standard pressure is chosen as exactly 1 atm (760torr). Together, these conditions are referred to as standard conditions or as standard temperature and pressure (STP). While there is nothing special about STP, some authors and some instructors find it convenient to use this short notation for this particular temperature and pressure. 

The law we need here that is applied to situations where the number of moles of gas and the volume stay constant can be derived from the Combined Gas Law ... 

Plan (1) Use the Combined Gas Law to calculate the volume that 1 L of the gas would have occupied... 

In order to work out reacting volumes of gases, we can use Gay-Lussac s law of combining volumes When gases react they do so in volumes which bear a simple ratio to each other and to the volumes of the products if they are gases, all measurements of volume being at the same temperature and pressure . 

B Here we are not dealing with gaseous reactants the law of combining volumes cannot be used. From the ideal gas equation we determine the amount of N2(g) per liter under the specified conditions. Then we determine the amount of Na(l) produced simultaneously, and finally the mass of that Na(l). 

A The law of combining volumes permits us to use stoichiometric coefficients for volume ratios. 

C is a temperature higher than STP. This condition increases the 1.00-L volume that contains 0.44 g of the mixture at STP. We calculate the expanded volume with the combined gas law. 

Combined gas law Amount (n) Pressure, volume, and temperature (K) (PiVO/Tj = (P2V2)/T2... 

It is possible to combine Avogadro s law and the combined gas law to produce the ideal gas equation, which incorporates the pressure, volume, temperature, and amount relationships of a gas. The ideal gas equation has the form of... 

The presence of two volumes (or two temperatures) is a very strong indication that we will need to use the combined gas law. To use this gas law we need subscripts to differentiate the different volumes and temperatures. Label one volume Vj and the other V2. It does not matter which volume we label 1 or 2 as long as we label all associated variables with the same subscript. 

For example, suppose a 5.0-L bottle of gas with a pressure of 2.50 atm at 20°C is heated to 80°C. We can calculate the new pressure using the combined gas law. Before we start working mathematically, however, let s do some reasoning. The volume of the bottle hasn t changed, and neither has the number of moles of gas inside. Only the temperature... 

Most gas law experiments use either the combined gas law or the ideal gas equation. Moles of gas are a major factor in many of these experiments. The combined gas law can generate the moles of a gas by adjusting the volume to STP and using Avogadro s relationship of 22.4 L/mol at STE The ideal gas equation gives moles from the relationship n = PV/RT. 

The values of P, T, and n may be used to determine the volume of a gas. If this volume is to be used with Avogadro s law of 22.4 L/mol, the combined gas law must be employed to adjust the volume to STE This equation will use the measured values for P and Talong with the calculated value of V. These values are combined with STE conditions (0°C (273.15 K) and 1.00 atm) to determine the molar volume of a gas. 

Use the combined gas law to calculate molar volume at room temperature and atmospheric pressure. 

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