Class 11 CBSE Ch 1 Some Basic Concept of Chemistry

5. Symbolic Representation

10. The term isotope is formed from the Greek roots isos (ἴσος "equal") and
topos (τόπος "place"), meaning "the same place"; thus, the meaning behind
the name is that different isotopes of a single element occupy the same
position on the periodic table.
All isotopes of a given
element have the same
number of protons but
different numbers of
neutrons in each atom.

11. The atomic number
and electronic
configuration is same
for all isotopes
Physical properties
like mass, density,
melting point, boiling
point are different
Isotopes are variants of a particular chemical element
which differ in neutron number, and consequently in
nucleon number.

12. Relative atomic mass is determined by the average atomic mass, or
the weighted mean of the atomic masses of all the atoms of a
particular chemical element found in a particular sample, which is
then compared to the atomic mass of carbon-12.
The prevailing IUPAC definitions (as taken from the "Gold Book") are:
atomic weight — relative atomic mass and
Relative atomic mass (atomic weight) — The ratio of the average mass
of the atom to the unified atomic mass unit. (amu)
Here the "unified atomic mass unit" refers to 1⁄12 of the mass of an
atom of 12C in its ground state.

13. So, Due to Chlorine RAM
we take its mass as 35.5

14.  The molecular mass (m) is the addition of masses of each element in a given
molecule: it is measured in Dalton's (Da or u). Different molecules of the same
compound may have different molecular masses because they contain different
isotopes of an element.
 The definition of molecular weight is most authoritatively synonymous with
molecular mass; however, in common practice, it is also highly variable as are the
units used in conjunction with it.
 Many common preparatory sources use g/mol and effectively define it as a
synonym of molar mass, while more authoritative sources use Da or u and align its
definition more closely with the molecular mass.
 Even when the molecular weight is used with the units Da or u, it is frequently as
a weighted average similar to the molar mass but with different units.
Example
 Molecular mass of methane, (CH4) = (12.011 u) + 4 (1.008 u) = 16.043 u
 Similarly, molecular mass of Ozone (O3) = 3 × atomic mass of oxygen
 = 3 x 16.00 u = 48.00 u

15. Watch Example and Gram molecular
mass in the next video slide

17. Calculate the molecular mass of ethanol, whose formula is CH3CH2OH. Among its
many uses, ethanol is a fuel for internal combustion engines.
Given: molecule Asked for: molecular mass
Strategy:
 A Determine the number of atoms of each element in the molecule
 B Obtain the atomic masses of each element from the periodic table and multiply
the atomic mass of each element by the number of atoms of that element.
 C Add together the masses to give the molecular mass.
 Solution: Taking the atomic masses from the periodic table, we obtain
 2 × atomic mass of carbon = 2 atoms ( 12 .011 amu atom ) = 24.022 amu
6 × atomic mass of hydrogen = 6 atoms ( 1 .0079 amu atom ) = 6.0474 amu
1 × atomic mass of oxygen = 1 atom ( 15 .9994 amu atom ) = 15.9994 amu
 Adding together the masses gives the molecular mass:
 24.022 amu + 6.0474 amu + 15.9994 amu = 46.069 amu
Solved Question

18. The formula mass of a molecule (also known as formula weight) is the sum of the
atomic weights of the atoms in the empirical formula of the compound.
Formula weight is given in atomic mass units (amu).
Example and Calculation
The molecular formula for glucose is C6H12O6, so the empirical formula is CH2O.
The formula mass of glucose is 12+2(1)+16 = 30 amu.
Note: Empirical formula we will learn in next coming few parts
Find the relative formula mass of carbon monoxide, CO?
The relative atomic mass of carbon is 12 and of oxygen is 16,
so the relative formula mass is: 12 + 16 = 28 amu
To find the relative formula mass of sodium oxide, Na2O,
you multiply the relative atomic mass of
sodium times its subscript and add the value to the relative atomic mass of oxygen:
(23 x 2) + 16 = 62
One mole of sodium oxide has a relative formula mass of 62 grams.

20. Calculate the formula mass of Ca3(PO4)2, commonly called calcium phosphate. This
compound is the principal source of calcium found in bovine milk.
 Given: ionic compound Asked for: formula mass
 Strategy:
 A Determine the number of atoms of each element in the empirical formula.
 B Obtain the atomic masses of each element from the periodic table and multiply the
atomic mass of each element by the number of atoms of that element.
 C Add together the masses to give the formula mass.
 Solution: Taking atomic masses from the periodic table, we obtain
 3 × atomic mass of calcium = 3 atoms ( 40 .078 amu atom ) = 120.234 amu
2 × atomic mass of phosphorus = 2 atoms ( 30 .973761 amu atom ) =61.947522 amu
8 × atomic mass of oxygen = 8 atoms ( 15 .9994 amu atom ) = 127.9952 amu
 Adding together the masses gives the formula mass of Ca3(PO4)2:
 120.234 amu + 61.947522 amu + 127.9952 amu = 310.177 amu
Solved Question

21. Try to find out the answers for following