Atmospheric Pressure Pressure is equal to a force per area. The gases in the air exert a pressure called atmospheric pressure. Atmospheric pressure is a result of all of the collisions that result from air particles bumping into things as they move around. Atmospheric pressure is measured with a barometer.

Units for Pressure mmHg (millimeters mercury) or torr 1.000 atm = 760.0 mmHg = 760.0 torr 1.000 atm = 101,325 pascal (Pa) 1.000 atm = 14.69 pounds per square inch (psi) SI UNIT! USED IN ENGINEERING!

Gases can be described using the following four variables: Properties of Gases Gases can be described using the following four variables: V = volume of the gas (liters, L) P = pressure (atmospheres, atm) T = temperature (Kelvin, K) n = amount (moles, mol) UNITS

Reading Quiz Gas Laws What is pressure? Convert 687 torrs into atmospheres. If the temperature (T) and number of moles (n) are constant, what happens to volume (V) as pressure (P) increases? What if P decreases?

ANSWERS P & V are ? P V related! P V F A indirectly Pressure (P) is force per unit area 687 torr 1 atm = 0.904 atm 760 torr If T is constant… as P increases, V decreases! as P decreases, V increases! P & V are ? related! P V indirectly P V

P may change and V may change, but their product stays the same! Boyle's Law P1 V1 = P2 V2 P may change and V may change, but their product stays the same!

Boyle's Law P1 V1 T & n constant

P2 V2 T & n Boyle's Law If pressure increases, volume decreases! constant If pressure increases, volume decreases! (when temperature’s constant)

V2 P2 T & n Boyle's Law If pressure decreases, volume increases! constant If pressure decreases, volume increases! (when temperature’s constant)

Inverse and Direct Proportions… which is which? B

Inversely Proportional Ex: Boyle’s Law (V as a function of P) P (torr) V (mL) 100 4560 200 2280 300 1520 400 1140 500 912 600 760 700 651 800 570 900 507 1000 456 1100 414 1200 380 1300 351 1400 326 1500 304

Boyle’s Law at Different Temperatures (P as a function of V)

INVERSE RELATIONSHIPS As one variable goes up, the other goes down! Produces a curved graph… Multiplying the two variables equals a constant. P V T constant P1V1 = P2V2

Directly Proportional Ex: Charles’ Law (V as a function of T) T (K) V (mL) 50 100 200 150 300 400 250 500 600 350 700 800 450 900 1000 550 1100 1200 650 1300 1400

DIRECT RELATIONSHIPS T As one variable goes up, so does the other! V Produces a straight line graph… Dividing the one variable by the other equals a constant. V1 V2 T1 T2 =

Want Practice?: V (L) n (mol) 5 10 15 20 1.0 25 30 35 40 45 50 55 60 65 70 75 Complete the following data table for Avogadro’s Law (Volume is constant) Graph your data (properly). Are V & n inversely or directly related? Why?

Directly Proportional (V as a function of n) Avogadro’s Law: Directly Proportional (V as a function of n) V (L) n (mol) 5.0 0.250 10. 0.500 15 0.750 20. 1.00 25 1.25 30. 1.50 35 1.75 40. 2.00 45 2.25 50. 2.50 55 2.75 60. 3.00 65 3.25 70. 3.50 75 3.75 V1 V2 n1 n2 =