1. A Level Chemistry
Measuring moles of GASES by measuring their volume, pressure and temperature
PV = nRT

2.  molecules move freely (no inter-molecular forces) and randomly
FACTORS AFFECTING THE VOLUME OF A GAS
Gas
 molecules move freely (no inter-molecular forces) and randomly
 gas mols will always DIFFUSE to fill whatever volume is available.
Volume (V) of a gas depends on the :
1. number of moles (n) of gas
2. pressure (P) applied to the gas
3. temperature (T) of the gas
Volume does NOT depend on the type of gas!

3. AVOGADRO’S LAW: V n
V  n
provided P and T fixed
V = k1n

4. BOYLE’S LAW: P V
V  1/P
provided n and T fixed
PV = k2

5. CHARLES’S LAW: V  T V = k3T provided n and P fixed
0oC
273K V
T /K
provided n and P fixed
V  T
and T measured in KELVIN
V = k3T
T (/K) = T(/OC) + 273

6. the IDEAL GAS EQUATION PV = nRT
All 3 laws can be combined into one equation
the IDEAL GAS EQUATION
PV = nRT
where "R" is called the
UNIVERSAL GAS CONSTANT
NB : Must use appropriate S.I. units
R = 8.31 J mol-1 K-1
VARIABLE V P T n
S.I. UNIT m3
Nm-2 (or Pa) K
Moles

7. Notes :
1. Nm-2 = Pascal = "Newtons per square metre"
(1 kg spread over 1m2  approx. 10Pa)
2. 1 kPa = 1000 Pa
3. 1m3 = 1000 dm3 (1000 litres) = cm3
4. Use PV = nRT to calculate n, given P,V and T
 calculate Mr if also know mass, m (/g)
Mr = m / n

8. IDEAL GAS EQUATION not perfect at all T and P for any gas
OK within experimental error provided :
a. P is not too high
b. T is not too low
c. Molecules are not too large.
ie equation less reliable if gas is close to liquefying
A truly "IDEAL" gas would OBEY the equation at ALL T and P.
NO SUCH GAS EXISTS!
He and H2 are the nearest because these show the weakest intermolecular forces

9. Q1. Calculate the number of moles present, if a
Q1 Calculate the number of moles present, if a gas exerts 110 kPa pressure at 20oC in a volume of dm3 P V n R T
PV = nRT
110 kPa
= Pa RT PV
n =
20.0 dm3
= m3 =
x 0.020
8.31 x 293
????
= moles
8.31 J mol-1 K-1
20oC
= 293 K

10. PV = nRT P V n R T 100 kPa = 100000 Pa RT PV n = 100 cm3 = 0.0001 m3 =
Q2 A sample of gas weighs 0.272g. It has a volume of 100cm3 at a temperature of 16oC and pressure 100 kPa. Calculate the number of moles of gas in the sample and hence the mass of one mole of the gas. P V n R T
PV = nRT
100 kPa
= Pa RT PV
n =
100 cm3
= m3 = x
8.31 x 289
????
= 4.16 x 10-3 moles
8.31 J mol-1 K-1
Mr = m/n =
0.272
4.16 x 10-3
16oC
= 289 K
= g

11. The End