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HL1-8.ppt Gas Laws HL Chemistry

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**Review – Avogadro’s Law**

Equal volumes of gases at the same T & P contain the same number of molecules. (which means that coefficients in a balanced equation can be ‘read’ as volumes of gases)

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Review – Molar Volume 1 mole of any gas takes up 22.4 dm3 of space at STP Standard Temperature (273K or 0oC) & Pressure (101 kPa or 1 atm) VSTP = dm3 / mol Which can be rearranged… n = VSTP / 22.4 dm3 mol

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Review – Molar Volume 1 mole of any gas takes up 22.4 dm3 of space at STP Standard Temperature (273K or 0oC) & Pressure (101 kPa or 1 atm) VSTP = dm3 / mol Which can be rearranged… n = VSTP / 22.4 dm3 mol

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**Other applications of Avagadro’s law &/or Molar Volume:**

Determine number of moles (or g) of a given volume of gas at STP. (1.4.2 # 3, part of 5, part of 7) Determine volume of a known quantity (mol or g) of a gas at STP (1.4.2 # 4, part of # 10) Calculate molar mass given information about moles (or g) and volume… and knowing molar volume (1.4.2 # 8) Determine density of a gas at STP (1.4.2 # 9)

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**The Ideal Gas Law Q – Can all gases be considered ‘ideal’?**

Most real gases act ideally under normal conditions. They tend to act less ideally when: They have high molar masses Why? They are at very high pressures Why? They are at very low temperatures Why?

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**Which of these gases would act the least ideally?**

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**The Ideal Gas Law PV=nRT where P = pressure in kPa**

V = volume in dm3 (Litres) n = moles of gas R = universal gas constant (8.314 kPa dm3 mol-1 K-1) ( if you’re using atm, which you won’t in IB…) T = temperature in K

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**Pressure - You want pressure in kPa…**

What do you do if it is given in mm Hg? What do you do if it is given in atm?

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The Ideal Gas Law As long as you can remember PV=nRT, you can derive all of the other gas laws. If the number of moles of gas is not changing, we can just solve for n. n=PV/RT Since the moles aren’t changing, we can set the right side of the equation equal to itself. P1V1/RT1= P2V2/RT2 and because R is the same… P1V1/T1= P2V2/T2 (combined gas law)

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If you have a problem where one of these variables is held constant, you can simplify the combined gas law by eliminating that variable: P1 V1 / T1 = P2 V2 / T2 P1 V1 = P2 V2 V1 / T1 = V2 / T2 P1 / T1 = P2 / T2

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**We now have this collection of gas laws:**

P1V1/T1=P2V2/T2 (Combined Gas Law) P1V1=P2V2 (Boyle’s Law) V1/T1=V2/T2 (Charles’ Law) P1/T1=P2/T2 (Gay Lussac’s Law)

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Gas Laws TIP #1: Always use K for temperatures because it is an “absolute” scale (no negatives)T Gas Laws TIP #2: If P or V are on both sides, the units don’t matter as long as they are the same on both sides. Gas Laws TIP #3: You must be VERY careful with your units when using the Ideal Gas Law!

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**Your Assignment: Read pp. 21-26 Do Ex 1.4.3 on pp. 25-26 # 1-10**

Identify which gas law you are using to solve the problem… then solve the problem . Show your work…. (at minimum include the set up using some form of a gas law)

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Charles Law Charles's law is an experimental gas law which describes how gases tend to expand when heated. It was first published by French natural philosopher Joseph Louis Gay-Lussac in 1802, although he credits the discovery to unpublished work from the 1780s by Jacques Charles.

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Charles Law T V The following is a more useful way of expressing this law.

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**Create a hypothesis for what happened here.**

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**Here’s what really happened.**

Workers used hot pressurized water to clean the tanker car. Tank was drained. Door was closed and tanker was left overnight. Temperature inside the car plummeted overnight. V/T=V/T (Temp goes down, Volume must go down also)

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