Solving Gas Law Problems

Gas law problems are phrased in many ways, but they can usually be grouped into two main types  

A citange in one of the four variables causes a change in another, while the two remaining variables remain constant. In this type, the ideal gas law reduces to one of the individual gas laws, and you solve for the new value of the variable. Units must be consistent, T must always be in kelvins, but R is not involved. Sample Problems 5.2 to 5.4 and 5.6 are of this type. (A variation on this type involves the combined gas law for simultaneous changes in two of the variables that cause a change in a third.) 

One variable is unknown, but the other three are known and no change occurs. In this type, exemplified by Sample Problem 5.5, the ideal gas law is applied directly to find the unknown, and the units must conform to those in R. 

These problems are far easier to solve if you follow a systematic approach  

The following series of sample problems applies the various gas behaviors. 

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When solving gas law problems using the combined gas law, the pressure and volume units do not have to be as indicated by the authors of the laws and by the ideal gas law they don t even have to be in the metric system. However, temperature must be in the Kelvin scale. Explain. 

Solving gas law problems using these formulas is a straightforward process of algebraic manipulation. Errors commonly arise from using improper units,... 

Boyle s Law Charles s Law Avogadro s Law Standard Conditions The Ideal Gas Law Solving Gas Law Problems... 

We use rearrangements of the ideal gas law such as this one to solve gas law problems, as you ll see next. The point to remember is that there is no need to memorize the individual gas laws. 

To avoid using the units of / as a fraction, we will use it as follows R = 0.0821 L atm K mol. Note carefully the units of R pressure is in atmospheres, temperature in kelvins, volume in liters, and n in moles. To use this equation for solving gas law problems you must pay strict attention to all units. 

Solve In analyzing and solving gas law problems, it is helpful to tabulate the information given in the problems and then to convert the values to units that are consistent with those fork (0.08206 L-atm/mol-K). In this case the given values are... 

As you ll see next, you can nse a similar approach to solve gas law problems. Thus, by keeping track of the initial and final values of the gas variables, you avoid the need to memorize the three individual gas laws. 

The Relationship Between Volume and Amount Avogadro s Law 156 Gas Behavior at Standard Conditions 156 The Ideal Gas Law 157 Solving Gas Law Problems 158... 

There are two ways to solve gas law problems by algebra, as shown in the preceding example, and by reasoning. Reasoning is based on the proportionality between the variables. In this case the variables are volume and temperature, which are directly proportional to each other. They move in the same direction if one goes up, the other goes up (and vice versa), as shown in Figure 4.11, when a sample of gas is heated. 

Deletion of the molar volume approach to gases The second edition had a chapter that used molar volume as the primary strategy for solving gas law problems. Most users preferred the ideal gas equation approach and omitted the alternative chapter. Therefore, we have omitted the alternative chapter. However, Thomson Custom Publishing will provide it to users on request. 

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