Page 45 - Understanding NCERT Science 09

(c) Formula of sodium nitrate:
following formulae:
(i) Al (SO )
2 4 3
(ii) CaCl
2
(iii) K SO
2 4
(iv) KNO
3
Formula : NaNO (v) CaCO .
3 3
3. What is meant by the term
(d) Formula of calcium hydroxide: chemical formula?
4. How many atoms are present in a
(i) H S molecule and
2
3–
(ii) PO ion?
4
Formula : Ca(OH) 3.5 Molecular Mass
2
Note that the formula of calcium 3.5.1 MOLECULAR MASS
hydroxide is Ca(OH) and not CaOH . We use
2 2
brackets when we have two or more of the In section 3.2.2 we discussed the concept of
same ions in the formula. Here, the bracket atomic mass. This concept can be extended
around OH with a subscript 2 indicates that to calculate molecular masses. The molecular
there are two hydroxyl (OH) groups joined to mass of a substance is the sum of the atomic
one calcium atom. In other words, there are masses of all the atoms in a molecule of the
two atoms each of oxygen and hydrogen in substance. It is therefore the relative mass of
calcium hydroxide. a molecule expressed in atomic mass units (u).

(e) Formula of sodium carbonate:
Example 3.1 (a) Calculate the relative
molecular mass of water (H O).
2
(b) Calculate the molecular mass of
HNO .
3
Solution:
Formula : Na CO
2 3
(a) Atomic mass of hydrogen = 1u,
In the above example, brackets are not needed oxygen = 16 u
if there is only one ion present.
So the molecular mass of water, which
(f) Formula of ammonium sulphate: contains two atoms of hydrogen and one
atom of oxygen is = 2 × 1+ 1×16
= 18 u
(b) The molecular mass of HNO = the
3
atomic mass of H + the atomic mass of
Formula : (NH ) SO N+ 3 × the atomic mass of O
4 2 4 = 1 + 14 + 48 = 63 u
Q 1. Write down the formulae of 3.5.2 FORMULA UNIT MASS
uestions

(i)
sodium oxide
(ii)
aluminium chloride
The formula unit mass of a substance is a sum
sodium suphide
(iii)
of the atomic masses of all atoms in a formula
magnesium hydroxide
(iv)
2. Write down the names of unit of a compound. Formula unit mass is
calculated in the same manner as we calculate
compounds represented by the the molecular mass. The only difference is that
34 SCIENCE

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