Prentice Hall ©2004 Ppt17b Ppt 17b, Continuation of Gases & Gas Laws 1. Individual Gas Laws Combine to Form the Ideal Gas Law. 2. Ideal Gas Law Problems-I Find one variable if the other three are known (“one state” problem) Problems in which the state of a gas changes (“two state” problem). 3. Density of Gases Ideas & Calculations Can use Ideal Gas Law here (or memorize variant) 1

Prentice Hall ©2004 Ppt17b Important Comment In all previous laws, the proportionality constant, k 1, is not a fundamental constant of nature; it would depend on the conditions [the values of the other variables being held constant] 2

Prentice Hall ©2004 Ppt17b The Ideal Gas Law01 Ideal gases obey an equation incorporating the laws of Charles, Boyle, and Avogadro: The gas constant R = L·atm·K –1 ·mol –1 STP conditions are K and 1 atm pressure 1 mole of an ideal gas occupies L at STP (molar volume = V of 1 mol) [same for all gases, conditions] 3

Prentice Hall ©2004 Ppt17b The Ideal Gas Law02 Sulfur hexafluoride (SF 6 ) is a colorless, odorless, very unreactive gas. Calculate the pressure (in atm) exerted by 1.82 moles of the gas in a steel vessel of volume 5.43 L at 69.5°C. What is the volume (in liters) occupied by 7.40 g of CO 2 at STP? 4

Prentice Hall ©2004 Ppt17b What if more than one variable changes? Oxygen gas is normally sold in 49.0 L steel containers at a pressure of atm. What volume would the gas occupy if the pressure was reduced to 1.02 atm and the temperature raised from 20 o C to 35 o C? 5

Prentice Hall ©2004 Ppt17b Example #2 An inflated balloon with a volume of 0.55 L at sea level, where the pressure is 1.0 atm, is allowed to rise to a height of 6.5 km, where the pressure is about 0.40 atm. 1) Assuming that the temperature remains constant, what is the final volume of the balloon? 2) Assuming that the temperature at sea level is 22°C and the temperature at 6.5 km is -28°C, what is the final volume of the balloon? 6

Prentice Hall ©2004 Gases Worksheet-I Practice Proportional relationships / calculations Ppt17b7

Prentice Hall ©2004 The Ideal Gas Law--Applications04 Recall: At the same T and P, equal volumes of gases contain equal numbers of molecules.  V  n (P, T constant) a fixed value (at a given P & T, for any gas!)  Implication?  The concentration of gas particles is “fixed” at a given T and P! 8Ppt17b

Prentice Hall ©2004 At the same T & P, [gas] is “fixed” (sample size does not matter) (identity of the gas does not matter) …same number of particles per L Same T & P  … He Ar H2H2 He Ppt17b9

Prentice Hall ©2004 Density of a gas at fixed T and P does depend on gas identity (molar mass)! Density is “number density” x mass per particle Which equals concentration x molar mass! One could substitute in for n/V here to get an equation that relates d, P, T, and MM, – but I don’t think it’s really “worth it” (see answer key comments) This slide is intended to make a conceptual connection, not give you another equation to memorize! 10

Prentice Hall ©2004 Summary (of prior few slides) : At the same T & P, concentration is constant, but density varies with MM …different densities for different gaseous substances, because mass Same # particles / L, but… per particle (and thus per mole) varies! He Ar H2H2 He Greatest density b/c greatest MM 11

Prentice Hall ©2004 Practical Application: Will gases (or balloons filled with them) float or sink (in air)? MM is the key! Air is about 80% N 2 and 20% O 2  MM(air)  29 g/mol Substance or Mixture Molar Mass (g/mol) Floats or sinks in air? (at same T, P) H2H2 2.0floats He4.0floats Air~29Neither ( but balloon would sink b/c the rubber makes d balloon slightly > d air ) CO 2 44sinks Quiz: Would a neon balloon float or sink? Argon? Ppt17b12

Prentice Hall ©2004 The bottom line? Density of a gas depends on molar mass, T, & P, but not on “amount” Since amount does not matter, I prefer to PICK A CONVENIENT AMOUNT* to solve problems that relate to gas density and molar mass!  No need to memorize or derive separate equations (although if you want to do so, more power to you!) * 1 L of gas, if density is given; 1 mol of gas, if molar mass (or formula) is given Ppt17b13

Prentice Hall ©2004 The Ideal Gas Law—Applications (Examples) What is the molar mass of a gas with a density of 4.07 g/L at a pressure of 3.42 atm and 35°C? What is the density of uranium hexafluoride, UF 6, (MM = 352 g/mol) under conditions of STP? To what temperature must a sample of CO 2 be raised at 1.97 atm in order for the density to become 3.38 g/L? Ppt17b14