A Reminder… assume ideal We that we live in an world where: Gas particles have no mass Gas particles have no volume Gas particles have elastic collisions These assumptions are used when trying to calculate the AMOUNT of a gas we have!

Why are these assumptions important? PV = nRT Image source: thefreedictionary.com

PV = nRT P V n R T The Ideal Gas Law RESSURE OLUME MOLES OF GAS GAS CONSTANT EMPERATURE Image source: popartuk.com

The MysteRious R 62.4 mmHg · L mol · K 8.31 kPa · L mol · K R is a constant (doesn’t change). Number value of R depends on other units. Units of R are a combination of many units. 62.4 mmHg · L mol · K 8.31 kPa · L mol · K 0.0821 atm · L mol · K Image source: toysrusemail.com

Ummm… What? PV = nRT P V R = n T (kPa) (mm Hg) (L) (atm) R = (mol) (K) Solve for R: P V R = n T Plug in units: (kPa) (mm Hg) (L) (atm) R = (mol) (K)

Gas Laws, Gas Laws Everywhere! T1 = V2 T2 Charles' Law Boyle's Law P1 x V1 = P2 x V2 P1 V1 P2 V2 = T1 T2 Combined Gas Law Ideal Gas Law Used with CHANGING CONDITIONS P V = n R T Used with only ONE SET OF CONDITIONS

When to Use PV = nRT Calculating amount of gas in moles Calculating P, V, or T if moles of gas are known. IMPORTANT! We must have 3 out of 4 pieces of information: P V n T

Practice with the Ideal Gas Law A gas sample occupies 2.62 L at 285ºC and 3.42 atm. How many moles are present in this sample?

But Let’s Be Practical… We don’t usually measure in moles! We usually measure quantities in GRAMS! PV = nRT PVM = gRT

PVM = gRT P V M g R T RESSURE OLUME OLAR MASS OF GAS (g/mol) RAMS OF GAS GAS CONSTANT EMPERATURE Image source: popartuk.com

Practice with the Ideal Gas Law A balloon is filled with 0.2494 g of helium to a pressure of 1.26 atm. If the desired volume of the balloon is 1.250 L, what must the temperature be in ºC?

PV=nRT vs. PVM=gRT Use PV=nRT when: Use PVM=gRT when: You are given moles in the problem. You are searching for moles as an answer. Use PVM=gRT when: You are given grams in the problem. You are searching for grams as an answer.