Ideal Gases

Question 1

SI unit of pressure

Answer 1

Pascals (Pa)

Question 2

alternative units for pressure

Answer 2

1 Pa = 1 Nm-2 = 1 kgm-1s-2

**USEFUL: 1 Pa = 1 Jm-3

Question 3

kPa and bar conversion

Answer 3

100 kPa = 1 bar

100,000 Pa = 1 bar

Question 4

atmospheric pressure in kPa, atm, mmHg, Torr

Answer 4

about 100 kPa
1 atm
760 mmHg = 760 Torr

Question 5

volume SI unit

Answer 5

m3

Question 6

m3, dm3 and cm3 conversion

Answer 6

10^6 cm3 = 10^3 dm^3 = 1 m^3

1000 cm3 = 1000 mL = 1 dm3 = 1L

Question 7

Temperature Celsius to Kelvin

Answer 7

T = (ø + 273.15) K

Question 8

define ideal gas

Answer 8

neglect attractions between gas molecules due to intermolecular forces, and the size of the molecules

particles are:

• point masses (no V)
• move in straight lines
• collide elastically (constant total Ek)
• are not attracted to one another

Question 9

Boyle’s Law

Answer 9

at constant temperature, the volume of a gas is inversely proportional to applied pressure.

PV = constant 
P1V1 = P2V2 

generally obeyed at low pressures

Question 10

Charles’ Law

Answer 10

volume of a given amount of gas at constant pressure is directly proportional to temperature (K)

V/T = constant 
V1/T1 = V2/T2

Question 11

the general gas law

Answer 11

combines Boyle’s and Charles’ laws

(P1V1)/T1 = (P2V2)/T2

Question 12

the amount-volume relationship

standard volume occupied by most gases

Answer 12

V is proportional to n

1 mol of most gases occupies 22.4L at 0˚C, 1atm

Question 13

The Ideal Gas Law

Answer 13

PV = nRT

P in Pa
V in m^3
T in K

R = 8.314 JK-1mol-1

real gases obey this law most closely if the pressure is lower than 200kPa

Question 14

conc of gas calculation

Answer 14

c(gas) = n/V = P / RT

in molm-3

Question 15

density of a gas calculation

Answer 15

d(gas) = m/V
= nM / V
= PM / RT (this is basically c(gas) x M)

in gm-3

Question 16

Dalton’s law

Answer 16

the total pressure exerted by a mixture of ideal gases is equal to the sum of partial pressures.

P(total) = P(A) + P(B)

Question 17

define partial pressure

Answer 17

the pressure a gas would exert if it occupied the volume alone.

Question 18

ideal gas equation for partial pressures

Answer 18

P(A) = n(A) x RT/V

basically PV = nRT rearranged, using pressure and number of moles of the gas A

partial pressure directly proportional to number of moles of gas

Question 19

Application of Dalton’s law - collecting gas over a volatile liquid

Answer 19

for H2O, assume the gas is saturated, so P(H2O) = the saturated vapour pressure of water at the temperature of the water.

P(total) = P(Gas) + P(H2O)

Question 20

relationship between partial gas and total gas pressure

Answer 20

P(A) = X(A) x P(tot)

where X(A) is the ratio of the number of moles: 
X(A) = n(A) / n(tot)