RELATIVE MOLECULAR MASS: -
The relative atomic mass or the relative molecular mass of an element is the number of times one mole of the element is as heavy as one-twelfth (1/12) the mass of one atom of carbon -12. It has no unit.
The relative molecular mass of a compound is the sum of the masses of all the atoms present in one molecule of the compound. e.g.
For NaCl, the relative molecular mass= (23 +35.5) = 58.5
For ethanol = C2H5OH (carbon=12, H=1, O =16)
The relative molecular mass of ethanol
= C2H5OH
(12×2) + (1×5) + (16) + (1)
24 + 5 + 16 +1 = 46
activities
1. Calculate the relative molecular mass of the following compounds
a. NaOH b. CaCO3 c. Al2(SO4)2
[ Na = 23, O =16, S = 32, Ca = 40, Al = 27, C = 12, H = 1]
Solution
a. NaOH
(Na x 1) + (O x 1) + (H x 1)
(23 x 1) + (16 x 1) + (1 x 1)
23 + 16 + 1 =
40
b. CaCO3
(Ca x 1) + (C x 1) + (O x 3)
(40 x 1) + (12 x 1) + (16 x3)
40 + 12 + 48 =
100
c. Al2(SO4)3
(Al x 2) + (S x 3) + (O x 12)
(27 x 2) + (32 x 3) + (16 x 12)
54 + 96 + 192 =
342
THE MOLAR MASS
This is the relative molecular mass expressed in grams. That is, expressing the molecular mass of a compound in grammes.
E.g. the molar mass of ethanol is 46gmol-1
In 12g of carbon-12, there are 6 × 1023 atoms of carbon. This is one mole of carbon -12.
THE MOLE CONCEPT
The mole is the amount of a substance that contains as many elementary particles as there are atoms in 12 g of C-12.
Carbon -12 isotope has been taken as an arbitrary standard, and it contains 6×1023 particles of C-12 atoms and it weighs 12grammes and so the amount of any substance that contains 6× 1023 is equal to 1 mole of the substance, and the mass will be equivalent to the molar mass of the substance.
A mole of any substance is the amount of that substance which contains 6× 1023 particles of that substance e.g. One mole of ethanol has a mass of 46g and contains 6× 1023 ethanol molecules.
The symbol of any element or the formular of any compound represents one mole of the element or compound. For example,
the symbol Na represents 1mole of sodium atom
Na+ represents 1mole of sodium ions
O2 represents 1mole od oxygen molecule
H2O represents 1mole of water molecule
and all the examples above contain Avogadro's number of particles that is, 6× 1023.
NOTE: The relative molecular mass has no unit, but the molar mass of any substance is expressed in grammes per mole (g/mol).
This number 6× 1023 is known as Avogadro’s number or Avogadro’s constant
Calculations on mole concept
1. How many moles are there in 20g of sodium atoms (Na)
Solution
1mole of Na =23 (since the symbol of an element represent 1mole)
Number of moles = mass given
molar mass (relative atomic/molecular mass)
i.e mass give = 20g, relative atomic mass of sodium = 23
number of moles = 20 = 0.87mols
23
the unit of the mole is mol
2. how many moles are there in 25g of CaCO3
Solution
1 mole of CaCO3= 40 + 12 + (16 x 3) = 100
i.e, 1mole of CaCO3 =100g
25g of CaCO3 will contain
number of moles = mass given 25 = 0.25mols
molecular mas 100
PERCENTAGE COMPOSITION OF A COMPOUND
The percentage composition of an element in a compound is calculated by dividing the total mass of the element in the compound multiply by 100.
Example
1.To calculate the percentage composition of the elements in ethanol whose molecular formula is C2H5OH, given the relative atomic masses of carbon, hydrogen and oxygen. [C= 12, H= 1, and O=16]
First calculate the molar mass of C2H5OH
Then determine the masses of C H and O present.
Mass of carbon= 12×2=24g
Mass of hydrogen= 6×1 = 6g
Mass of oxygen = 16× 1 = 16g
(12×2) +(1×6) + (16) = 46gmol-1
Molar mass of C2H5OH= 46g
Therefore, % of C = 24 × 100 = 52.17%
46
% of H = 6 × 100 = 13.04%
46
% of O = 16 × 100 =
34.78%
46
percentage composition of the elements in sodium hydroxide (Na2CO3) [Na= 23, O=16, C=12]
first you calculate the molecular mass of Na2CO3
(Na x 2) + (C x 1) + (O x 3)
(23 x 2) + (12 + 1) + (16 x 3)
46 + 12 + 48 = 106
% of Na in Na2CO3 = 46 x100 = 43.40%
106
% of C in Na2CO3 = 12 x100 = 11.32%
106
% of O in Na2CO3 = 48 x100 = 45.28%
106
OBJECTIVE QUESTIONS
1. what is the percentage composition of Sulphur in Al2(SO4)3
a.
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