Avagadro's law as it pertains to gases
May 19­10:06 PM
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Avagadro's Law as it pertains to gases states that:
Avagadro's law as it pertains to gases
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
May 19­10:06 PM
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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.
May 19­10:06 PM
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­ 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
May 19­10:06 PM
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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 amount of moles present by the volume of one mole according to the conditions the questions sets out, Whethere R.T.P or S.T.P.
1. given mass = Number of moles
Molar mass of gas
remember molar mass of gases is more than that of seperate atom
2. Number of moles x S.T.P or R.T.P = volume of that amount of moles
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
May 19­10:06 PM
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Use the information provided to answer the questions below:
i. What volume ism occupied by 8g of hydrogen at S.T.P ?
answer:
1mole of Hydrogen H2 has a mass of (2 x 1) =2g
2g of Hydrogen occupy 22.4 dm3 given it is S.T.P
8g of Hydrogen occupies 8.0 x 22.4 dm3 given it is s.t.p
2.0
=89.6 dm3
Now do these:
A. How much volume does 32g of Oxygen occupy at R.T.P?
B. How much volume does 280g of Chlorine occupy at S.T.P?
C. How much volume does 220g of Carbon dioxide gas occupy at R.T.P?
May 25­7:19 PM
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Jun 3­6:15 PM
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Calculating number of moles given volume:
Given volume present = number of moles
Volume 1 mole occupies present
at R.T.P or S.T.P
How many moles are present if there is a 36dm3 sample of Iodine gas at R.T.P?
36dm3 = 1.5 Moles
24dm3
How many moles are present of a 12dm3 sample of Chlorine gas at R.T.P?
?
12dm3 = 0.5 moles
24dm3
Jun 3­7:00 PM
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Calculating mass given the volume:
What mass of oxygen gas will occupy a volume of 96dm3 if it is at Room temperature and pressure?
Step1. First calculate the number of moles present, by dividing the given volume by the volume 1 mole occupies at R.T.P:
= Number of moles
Given volume Volume at R.T.P or S.T.P
96dm3 = 4 moles of Oxygen gas present
24dm3
Step2. Multiply the number of moles present by the mass of one mole of the gas.
number of moles x mass of one mole = Mass present
Activity:
4moles x 32g = 128g of Oxygen
Remember the mass of one mole of the gas will be twice as much as 1 mole of separate atoms as oxgyen gas will consist of diatomic particles.
1. Calculate the mass of Chlorine gas which will occupy a volume of 264dm3 if it is at R.T.P?
2. Calculate the mass of Flourine gas(F2) which will occupy a volume of 156.8dm3 at S.T.P?
Remember: 1 mole of any gas at R.T.P occupies 24dm3 and at S.T.P occupies 22.4dm3.
Jun 3­7:09 PM
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