3. GAY-LUSSAC’S LAW OF COMBINING
VOLUMES
GAY-LUSSAC (1805) OBSERVED THAT A SIMPLE RELATION EXISTS
BETWEEN THE VOLUMES OF HYDROGEN AND OXYGEN, WHICH REACT
TOGETHER TO FORM WATER. THUS, ONE LITRE OF OXYGEN REQUIRES
TWO LITRES OF HYDROGEN TO FORM 2 LITRES OF WATER VAPOUR. SO,
HE FOUND THAT OXYGEN AND HYDROGEN REACT IN THE RATIO 1 : 2 (BY
VOLUME). WHEN HE EXTENDED HIS STUDY TO THE VOLUMES OF OTHER
REACTING GASES, HE NOTED SIMILAR SIMPLE RELATIONSHIPS.
CONSEQUENTLY, HE GENERALIZED THESE OBSERVATIONS AS THE LAW
OF COMBINING VOLUMES OF GASES.
4.
5. THE LAW MAY BE ILLUSTRATED BY THE FOLLOWING EXAMPLES INVOLVING GASES OR VAPOURS:
REACTION OF NITROGENAND HYDROGEN TO FORM AMMONIA
EXPERIMENTALLY IT IS NOTICED THAT ONE VOLUME OF NITROGEN COMBINED WITH THREE
VOLUMES OF HYDROGEN GIVES TWO VOLUMES OF AMMONIA AT THE SAME TEMPERATURE
AND PRESSURE
N2+3H2→2NH3
THUS, THE RATIO 1 : 3 : 2 IS SIMPLE.
6. AVOGADRO’S LAW
AMEDEO AVOGADRO IN 1811 PUT FORWARD A HYPOTHESIS BASED ON THE RELATIONSHIP
BETWEEN THE NUMBER OF MOLECULES IN EQUAL VOLUMES OF DIFFERENT GASES UNDER
SIMILAR CONDITIONS.
AVOGADRO’S LAW STATES THAT “EQUAL VOLUMES OF ALL GASES UNDER SIMILAR CONDITIONS
OF TEMPERATURE AND PRESSURE CONTAINTHE SAME NUMBER OF MOLECULES.”
THIS MEANS THAT ONE LITRE OF HYDROGEN CONTAIN THE SAME NUMBER OF MOLECULES AS
ARE PRESENT IN ONE LITRE OF OXYGEN IN ONE LITRE OF CHLORINE OR OF ANY OTHER GAS
PROVIDED THE VOLUMES OF ALL GASES ARE MEASURED AT THE SAME TEMPERATURE AND
PRESSURE.
7. RELATIVE ATOMIC MASS RELATIVE MOLECULAR MASS
• THE RELATIVE ATOMIC
MASS OR ATOMIC
WEIGHT OF AN
ELEMENT IS THE NO.
OF TIMES ONE ATOM
OF THE ELEMENT IS
HEAVIER THAN 1/12
TIMES OF THE MASS
OF AN ATOM OF
CARBON-12.
• THE ATOMIC MASS OF
AN ELEMENT
EXPRESSED IN GRAMS
IS CALLED GRAM
ATOMIC MASS.
• THE RELATIVE
MOLECULAR MASS OR
WEIGHT OF AN ELEMENT
OR A COMPOUND IS THE
NO. THAT REPRESENTS
HOW MANY TIMES ONE
MOLECULE OF THE
SUBSTANCE IS HEAVIER
THAN 1/12 OF THE MASS
OF AN ATOM OF CARBON-
12.
• THE MOLECULAR MASS
OF A SUBSTANCE
EXPRESSED IN GRAMS IS
CALLED GRAM
MOLECULAR MASS OR
MOLAR MASS.
8. MOLE CONCEPT
IT IS NOT PRACTILLY POSSIBLE TO FIND THE MASS OF A MINUTE
PARTICLE LIKE AN ATOM, A MOLECULE, OR AN ION ETC. THEREFORE, A
COLLECTION OF 6.022×10²³ ELEMENTARY PARTICLES CALLED MOLE IS
TAKEN FOR ALL PRACTICAL PURPOSES.
A MOLE IS THE AMOUNT OF PURE SUBSTANCE CONTAINING THE SAME
NUMBER OF CHEMICAL UNITS AS THERE ARE ATOMS IN EXACTLY 12
GRAMS OF CARBON-12.
AVAGADRO’S NUMBER IS DEFINED AS THE NUMBER OF ATOMS PRESENT
IN 12 GRAM OF C-12 ISOTOPE.
9. VAPOUR DENSITY AND ITS RELATION TO
RELATIVE MOLECULAR MASS
THE RELATIVE VAPOUR DENSITY OF A GAS IS THE RATIO BETWEEN THE
MASSES OF EQUAL VOLUMES OF GAS AND HYDROGEN UNDER THE SAME
CONDITIONS OF TEMPERATURE AND PRESSURE.
RELATIVE
V.D.=MASS OF VOLUME V OF THE GAS UNDER SIMILAR CONDITIONS
MASS OF VOLUME V OF HYDROGEN GAS UNDER SIMILAR CONDITIONS
ACCORDING TO AVOGADRO’S LAW VOLUMES AT THE SAME TEMPERATURE
AND PRESSURE MAY BE SUBSTITUTED BY MOLECULES.HENCE,
RELATIVE V.D.= MASS OF ONE MOLECULE OF GAS OR VAPOUR
MASS OF TWO ATOMS OF HYDROGEN
THE RELATIVE MOLECULAR MASS OF A GAS IS TWICE ITS VAPOUR
DENSITY.
10. EMPIRICAL FORMULA OF A COMPOUND
THE EMPIRICAL FORMULA OF A COMPOUND IS THE SIMPLEST
FORMULA, WHICH GIVES THE SIMPLEST RATIO IN WHOLE
NUMBERS OF ATOMS OF DIFFERENT ELEMENTS PRESENT IN
ONE MOLECULE OF THE COMPOUND.
FOR EXAMPLE,THE EMPIRICAL FORMULA OF HYDROGEN
PEROXIDE IS HO. IT INDICATES THE SIMPLEST RATIO (1 : 1)
BETWEEN THE HYDROGEN AND OXYGEN ATOMS IN ITS
MOLECULE WHEREAS ITS ACTUAL FORMULA IS H2O2 .
11. THE EMPIRICAL FORMULA MASS IS THE SUM OF ATOMIC MASSES OF VARIOUS
ELEMENTS PRESENT IN THE EMPIRICAL FORMULA.
THUS, FOR HYDROGEN PEROXIDE (H2O2), THE EMPIRICAL FORMULA MASS IS 1 +
16=17.
12. DETERMINATION OF MOLECULAR
FORMULA
STEPS TO FIND THE MOLECULAR FORMULA OF A COMPOUND :
(1) CALCULATE THE EMPIRICAL WEIGHT OF THE COMPOUND FROM ITS EMPIRICAL
FORMULA,
(2) DIVIDE ITS MOLECULAR WEIGHT BY EMPIRICAL WEIGHT WHICH GIVES THE
NUMBER (N).
(3) MULTIPLY THE EMPIRICAL FORMULA BY THIS NUMBER TO GET THE
MOLECULAR FORMULA.
MOLECULAR FORMULA = EMPIRICAL FORMULA X N
WHERE N=MOLECULAR FORMULA/EMPIRICAL FORMULA WEIGHT
FOR EXAMPLE, THE EMPIRICAL FORMULA OF HYDROGEN PEROXIDE IS HO AND ITS
EMPIRICAL FORMULA WEIGHT IS 1 + 16 = 17, BUT ITS MOLECULAR WEIGHT IS 34.
HENCE ITS MOLECULAR FORMULA IS 34/17 = 2 TIMES THE EMPIRICAL FORMULA,
I.E., H₂O₂.
13. STOICHIOMETRY
STOICHIOMETRY IS FOUNDED ON THE LAW OF CONSERVATION OF
MASS WHERE THE TOTAL MASS OF THE REACTANTS EQUALS THE TOTAL
MASS OF THE PRODUCTS, LEADING TO THE INSIGHT THAT THE RELATIONS
AMONG QUANTITIES OF REACTANTS AND PRODUCTS TYPICALLY FORM A
RATIO OF POSITIVE INTEGERS. THIS MEANS THAT IF THE AMOUNTS OF THE
SEPARATE REACTANTS ARE KNOWN, THEN THE AMOUNT OF THE PRODUCT
CAN BE CALCULATED. CONVERSELY, IF ONE REACTANT HAS A KNOWN
QUANTITY AND THE QUANTITY OF THE PRODUCTS CAN BE EMPIRICALLY
DETERMINED, THEN THE AMOUNT OF THE OTHER REACTANTS CAN ALSO BE
CALCULATED.
