Transcript Avagadro`s law as it pertains to gases

Avagadro's law as it
pertains to gases
Avagadro's Law as it pertains to gases states that:
Equal volumes of different gases under the same conditions of temeprature and
pressure contain the same (equal) numbers of molecules.
(i)1 vol of Hydrogen + 1 vol of Chlorine = 2 vols of Hydrogen
Chloride
(ii)2 vol of Hydrogen + 1 Vol of Oxygen = 2 Vols of water
(iii)1 Vol of Nitrogen + 3 Vol
= 2 Vols of NH3
Analysis of Hydrogen chloride gas showed that one volume of the gas
contained one atom of Hydrogen and one atom of chlorine. If the gas
consisted of single atoms you might deduce that 1 vol of Hydrogen and 1
volume of chlorine would give 1 volume of Hydrogen chloride, However two
Volumes of Hydrogen Chloride are produced ,they would have come from two
atoms of Hydrogen and two atoms of Chlorine. These atoms came from one
volume of each gas. Since atoms, are not divisible, this suggests that one
volume of each gas consists of 2 atoms combined. These gases are
described as being diatomic.
H2 + Cl2
2HCl
(ii)2 H2 + O2
2H2O
(iii)3H2 + N2
2NH3
Due to the very Low density of Gases (some lighter than air), Gases are
measured by their volume rather than their mass.
The volume of a gas which contains 1 mole of Molecules of that gas is known
as the molar volume of that gas. This volume is the same of all gases
measured at the same temperature and pressure.
The molar volume of gases is usually given under either of two sets of
Conditions:
1. Standard Temperature and Pressure (S.T.P) which is a temperature
of 273K (0 degrees celsius) and 101 kPa(1 atm).
The approximate volume of 1 mole of any gas at S.T.P is 22.4 dm3 or
22,400 cm3.
2. Room temperature and pressure (R.T.P) which is a temperature of
298K(25 degrees celsius) and 101 kPa(1 atm).
The Approximate volume of 1 mole of any gas at R.T.P is 24dm3 or
24,000 cm3.
Thus 1 mole of Oxygen gas
1. contains 6 x10x23 O2 Molecules
2. has a mass of (2 x 16g) = 32g
3. Contains 2 moles of oxygen atoms (2 x 6 x10x23)
4. Occupies a volume of 22.4 dm3 at S.T.P
5. Occupies a volume of 24 dm3 at R.T.P
It is important to remember that 1 mole of
oxygen gas contains twice as much oxgyen
than 1 mole of seperate oxgyen atoms and
has twice the weight of 1 mole of oxygen
atoms.
Review
- 1 mole of all gaseous substances occupies the same
volume if they are at the same temperature and pressure.
- S.T.P is 273k (0 degrees celsius) and 101kPa( 1 atm)
- The volume of 1 Mole of any substance at S.T.P is 22.4
dm3
- R.T.P is 298K or (25 degrees celsius) and pressure of 101
kPa(1 atm)
- The volume of 1 mole at R.T.P is 24 dm3
Why does 1
mole of gas
occupy less
space at S.T.P
than R.T.P?
Calculating volume given mass:
1. Convert the mass given to moles by dividing the given mass by the molar
mass of the gas. e.g 64g of Oxygen = 64 = 2 moles O2 gas
32
2. Multiply the moles of the gas by S.T.P or R.T.P according to which
conditions the question sets out, to find the volume that amount of moles
will occupy.
1. given mass
Molar mass of gas
remember molar mass
of gases is more than
that of seperate atoms!
= Number of moles
2. Number of moles x S.T.P or R.T.P = volume of that
amount of moles
Question: What volume is occupied by 16g of Hydrogen gas at
S.T.P?
1 mol of Hydrogen, H2, has a mass of (2 x 1g) = 2g
2g of Hydrogen occupy 22.4dm3 at S.T.P
8g of Hydrogen occupy 16g x 22.4 dm3 at S.T.P =89.6dm3
2g