Download The Law of Definite Proportions

Foundation Year
Laws of chemical combinations
Dr. Marwa Eid
1
Objectives
• The law of conservation of mass
• The law of definite proportions
• The law of multiple proportions
• The law of combining volumes(Gay Lussac’s law of gaseous
volumes)
• Dalton’s atomic theory
• Modern atomic theory
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The law of conservation of mass
• Lavoisier (France 1789) states that:
‘Mass can neither be created nor destroyed in a chemical reaction’
• This means that during a chemical reaction the sum of the masses of
the reactants are equal to the sum masses of products.
Mass of Reactants
=
Mass of Products
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The law of conservation of mass
9.2 g of sodium bicarbonate on reaction with 30 g of acetic acid (CH3COOH)
liberated 5.8 g of carbon dioxide gas into atmosphere. What is the mass of residue
left?
9.2 + 30 = X + 5.8
X = 39.2 – 5.8 = 33.4 g
It proves the law of conservation of mass
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The law of conservation of mass
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The law of conservation of mass
Because atoms must be equal on each side of the equation:
The correct answer is B
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The law of conservation of mass
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The Law of Definite Proportions
“Constant Composition”
Joseph Proust (France 1799)
“A given compound always contains elements in a certain proportion
by mass”
 The ratios by mass of the elements in that compound are fixed independent of
the origins or preparation of that compound.
 A compound is unique because of the specific arrangement and weights of the
elements which make up that compound. (GR)
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The law of definite proportions
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The law of definite proportions
Example (1):
H2O is always made up of 2 atoms of H and one atom of O.
The ratio of O to H in water is always 16:2 or 8:1.
Example (2):
KCl always contains one atom of K for every one atom of Cl In KCl,
potassium and chlorine always have a ratio of “39.09 to 35.45” or “1.1 to 1” by mass.
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The law of definite proportions
Example (3):
What is the experimental percent of oxygen in CO2 if 45 g of carbon reacted
completely with 110 gm of oxygen?
Solution
% = (Portion/total)*100
%O2 = (mass of O2/Total Reactant mass )* 100
%O2 = 110/(45+110) = 70.9%
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Example (4):
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The law of multiple proportions
“Proposed that the same elements that make up one compound could
also make up another compound”
•When the same two elements combine to form more than one
compound:
•X+ Y
G
X+Y
M
the ratios of the mass of one element in the first compound to its
mass in the second compound, (as it combines with the same mass of
the other element).
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The law of multiple proportions
Carbon combines with oxygen to form CO and CO2
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The law of multiple proportions
Example (1)
A sample of sulfur dioxide with a mass of 10.00 g contains 5.00 g of sulfur. A
sample of sulfur trioxide with a mass of 8.33 g contains 3.33 g of sulfur.
1. What is the mass of oxygen in the sample of sulfur dioxide?
2. What is the mass of oxygen in the sample of sulfur trioxide?
3. For a fixed mass of oxygen in each sample,
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The law of multiple proportions
Example (2)
In the carbon compounds ethane (C2H6) and ethene (C2H4), what is
the lowest whole number ratio of H atoms that react with the same
number of C atoms?
H:C
Compound 1
6:2 = 3:1
Compound 2
4: 2 = 2:1
Ratio
3:2
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The law of multiple proportions
Example (3)
Hydrogen and oxygen forms two compounds. The hydrogen content in these
compounds is 42.9% and 27.3%. Prove these compounds agree with the law
of multiple proportions.
Hydrogen
Oxygen
H:O
Compound 1
42.9 g
100-42.9 = 57.1 g
42.9/57.1 = 75.5
Compound 2
27.3 g
100-27.3 = 72.7 g
27.3/72.7 = 0.375 g
Ratio
H : O = 0.75 : 0.375 = 2: 1
• which proves the law of multiple proportions.
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Foundation Year
Laws of Chemical Combinations
Dr. Marwa Eid
18
Objectives
• The law of conservation of mass
• The law of definite proportions
• The law of multiple proportions
• The law of combining volumes(Gay Lussac’s law of
gaseous volumes)
• Dalton’s atomic theory
• Modern atomic theory
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The law of combining volumes
(Gay Lussac’s law of gaseous volumes)
“When gases react together to form other gases,
and all volumes are measured at the same
temperature and pressure:
The ratio between the volumes of the reactant
gases and the products can be expressed in simple
whole numbers”.
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The law of combining volumes
(Gay Lussac’s law of gaseous volumes)
Example:
1 Liter of hydrogen + 1 Liter of chlorine = 2 Liters of Hydrogen Chloride
2 Liters of hydrogen + 1 Liter of oxygen = 2 Liters of Water
3 Liters of hydrogen + 1 Liter of nitrogen = 2 Liters of Ammonia
Mass is always conserved; but the volume of a gas is not.
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The law of combining volumes
(Gay Lussac’s law of gaseous volumes)
Avogadro’s Hypothesis:
Equal volumes of different gases contain the same number of particles.
The particles of a gas may be atoms or molecules.
One liter of hydrogen = one liter of chlorine = one liter of hydrogen chloride in
terms of particles (read atoms or molecules)
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The law of combining volumes
(Gay Lussac’s law of gaseous volumes)
• Mathematical relationship for P, V, T and number of moles of gas
PV = nRT
n = number of moles = mass/ Mwt
R = ideal gas constant = 0.0821 L atm / mol K
P = pressure (atm)
V = volume in (L)
T = Temperature in (K)
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The law of combining volumes
(Gay Lussac’s law of gaseous volumes)
42 g of oxygen is heated to 100 o C. If the pressure is 1.3 atm, what is
the volume?
• P = 1.30 atm ,V = ? ,m = 42 g , n = 42g/32g/mol , n = 1.31 mol
• R = 0.082057 L atm / mol K , T = 100 +273= oC = 373 o K
• PV = nRT
• V= nRT/P = (1.31 * 0.082057 * 373)/ 1.3 =
V= 27.65 L
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The law of combining volumes
(Gay Lussac’s law of gaseous volumes)
What is the mass of 41 L of chlorine at 28 oC and 1.7 atm?
• P = 1.7 atm , V = 41 L, n = ? , m = (n)(g/mol) , R =0 .082057
• T = 28 oC = 301 oK
• PV = nRT
• n= PV/RT = (1.7*41)/ (0.082057*301) =
n= 0.0688
m = n*Mwt = 0.0688 * 35.5= 2.44 g
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The law of combining volumes
(Gay Lussac’s law of gaseous volumes)
• What pressure in atm will 1.36 kg of N2O gas exert when it is
compressed in a 25.0 L cylinder and is stored in an outdoor shed
where the temperature can reach 59°C in summer?
V = 25.0 L T = 59+273 = 332 K
P = ? atm
R = 0.0821 Latm/ mol-K
m = 1.36 kg = 1360 g converted to moles
n= mass/Mwt = 1360/ 44 = 30.9 mol
• PV = nRT
• P = 30.90 mol x 0.0821 L-atm x 332 K
= 33.7 atm
25.0 L
mol-K
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The Combined Gas Law
 The
number of moles is held constant
 Is used when you have a change in volume,
pressure, or temperature
P 1V 1
T1
=
P 2V 2
T2
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The Combined Gas Law
When measured at STP, a quantity of gas has a volume of 500 dm3.
What volume will it occupy at 0 oC and 93.3 kPa?
PV
PV
1 1
 2 2
T1
T2
P1 =
T1 =
V1 =
P2 =
T2 =
V2 =
101.3 kPa
273 K
500 dm3
93.3 kPa
0 oC + 273 = 273 K
X dm3
(101.3 kPa) x (500 dm3) = (93.3 kPa) x (V2)
273 K
273 K
(101.3) x (500) = (93.3) x (V2)
V2 = 542.9 dm3
The Combined Gas Law
A sample of gas exerts a pressure of 83,326 Pa in a 300 cm3 vessel at 25oC. What
pressure would this gas sample exert if it were placed in a 500 cm3 container at 50 oC?
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Dalton’s atomic theory
“: All matter consists of tiny particles, called these particles “atoms””
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Dalton’s atomic theory
Dalton’s original theory had seven postulates:
1. Elements are made up of tiny particles called
“atoms”.
2. Atoms are indivisible.
3. Atoms can neither be created nor destroyed.
4. Atoms of the same element are alike in their
mass and chemical properties.
Dalton’s atomic theory
5. Atoms of different element differ in their mass and chemical
properties.
6. Atoms of one element cannot be converted into atoms of
another element
7. Atoms of different elements always combine in simple, definite
and integral ratios to form compound atoms.
1. According to Dalton ,what is an atom ?
2. What are the properties of an atom according to
Dalton ?
3. What are the postulates of Dalton's atomic theory ?
According to Dalton atom is the smallest,
indivisible particle of an element that takes part
in a chemical reaction.
Modern atomic theory
Modern Theory of the Atom
1• Atoms are composed of three main subatomic particles: the electron,
proton, and neutron.
2• Most of the mass of the atom is concentrated in the nucleus of the
atom.
3• The protons and neutrons are located inside the nucleus, while the
electrons exist outside of the nucleus.
4• In stable atoms, the number of protons is equal to the number of
electrons.
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Modern atomic theory
The type of atom is determined by the number of
protons it has.
• The number of protons in an atom is equal to the
atomic number.
• The sum of the number of protons and neutrons in a
particular atom is called the atomic mass.
• Valence electrons are the outermost electrons.
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