Scan_Doc0099 - T Write the ideal gas law, collecting the...

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EXAMPLE 5.7 f: :' £8 : 7 ; 1l--7.0 mL 6~ 5 i 4i 3i 2; 1= It. ~ As pressure increases, the volume decreases. 5.3 The Ideal Gas Law 191 The basic assumption we make when using the ideal gas law to describe a change in state for a gas is that the equation applies equally well to both the initial and final states. In deal- ing with changes in state, we always place the variables that change on one side of the equal sign and the constants on the other. Let's see how this might work in several examples. Suppose we have a sample of ammonia gas with a volume of 7.0 mL at a pressure of 1.68 atm. The gas is compressed to a volume of 2.7 mL at a constant temperature. Use the ideal gas law to calculate the final pressure. Solution Where are we going? To use the ideal gas equation to determine the final pressure What do we know? .I PI = 1.68 atm .I VI = 7.0 mL .I P 2 = ? .I V 2 = 2.7 mL What information do we need? .I Ideal gas law 4.4 PV = nRT .I R = 0.08206 L . atmiK . mol How do we get there? What are the variables that change? P, V What are the variables that remain constant? n,R,
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Unformatted text preview: T Write the ideal gas law, collecting the change variables on one side of the equal sign and the variables that do not change on the other. PV = nRT .7' '\ Change Remain constant Since nand T remain the same in this case, we can write PI VI = nRT and P 2 V 2 = nRT Combining these gives PIV I = nRT = P 2 V 2 or P I V I =P 2 V 2 We are given PI = 1.68 atm, VI = 7.0 mL, and V 2 = 2.7 mL. Solving for P 2 thus gives ( VI) (7.0 mt:) P 2 =-PI =---1.68 atm = 4.4 atm V 2 2.7 mt: Reality Check: Does this answer make sense? The volume decreased (at constant tem-perature), so the pressure should increase, as the result of the calculation indicates. Note that the calculated final pressure is 4.4 atm. Most gases do not behave ideally above 1 atm. Therefore, we might find that if we measured the pressure of this gas sample, the observed pressure would differ slightly from 4.4 atm. SEE EXERCISES 5.53 AND 5.54...
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This note was uploaded on 12/13/2010 for the course CHEM 2301 taught by Professor Bill during the Spring '10 term at South Texas College.

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