Choose the correct answers for each of the following questions.
1. Which one of the following group is comprised of the elements
(a) Mercury, Water, Ammonia
(b) iodine,
Tin, Iron
(c) Copper, Aluminium, Methane
(d) Coal, Smoke, Fog
2. Which one of the following can be
broken down into simpler substances.
(a) Ammonia
(b) Oxygen
(c) Sulphur
(d) Iron filling
3. The gram molecular mass of HNO3
is
(a) 60
(b) 100
(c) 63
(d) 98
4. Which of these molecules is not a
compound:
(a) N2O
(b) N2
(c) NO
(d) NO2
5. Which of the following is equal to
two moles of water (H2O)
(a) 1.084 x 1025 molecules
(b) 6.022 x 1023
molecules
(c) 1.204 x 1024 molecules
(d) 1.806 x 1024
molecules
6. A compound contains
(a) Different kinds of atoms mixed together
(b) the same kind of atoms mixed together
(c) Different kinds of atoms chemically
combined together
(d) The same kind of atoms chemically
combined together
7. Which one of the following is an
example of triatomic molecule
(a) CO2
(b) O2
(c) CH4
(d) NH3
8. Hydrogen gas is
(a) A monoatomic gas
(b) A mixture of hydrogen atoms
(c) A diatomic gas with each molecule made up
of two atoms
(d) A diatomic atom made up of two molecules
9. Which of the following compounds has
both empirical and molecular formula identical
(a) Benzene (C6H6)
(b) Hydrogen-per-Oxide (H2O2)
(c) Water
(H2O)
(d) Glucose (C6H12O6)
10.
Which
one of the following is a homogeneous mixture
(a) Smoke
(b) Air
(c) Fog
(d) Smog
Exercise Short Questions
How many elements are present in each of the following?
HF and HCl, Co and CO, Si and SiO2, POCl2 and POCl3
Solution
|
(a) In HF there are two elements while HCl also consists of two elements. |
(b) Co is one element while in CO there are two elements. |
|
(c) Si is one element while in SiO2 there are two elements. |
(d) In POCl2 there are three elements while in POCl3 there are also three elements. |
Cm is the chemical symbol for curium, named after the famous scientist Madam Curie. Why wasn’t the symbol C, Cu or Cr used instead?
The symbols
of elements should be unique so that they can be easily identified. For this
reason, C, Cu, and Cr are not the symbols of Curium because C is the symbol of
Carbon, Cu is the symbol of Copper while Cr is the symbol of Chromium.
What is the atomic number of an element? How does it differ from the mass number
Note: The answer
to this question is already attempted in the ”Short question’s answers” section of
Class 9 Chemistry Notes.
Differences between atomic number and mass number
|
Atomic number |
Mass Number |
|
The
number of protons present in the nucleus of an atom is called the atomic number. |
The sum
of the number of protons and neutrons present in the nucleus of an atom is called the mass number. |
|
The atomic number is represented by the capital letter Z. |
The atomic number is
represented by the capital letter A. |
|
It
is written at the subscript position of the symbol of the element i.e. AXZ. |
It is
written at the superscript position of the symbol of element i.e. AXZ |
|
The modern periodic table is arranged on the basis of the increasing order of atomic
numbers. |
The old periodic table was
arranged on the basis of the increasing order of atomic masses. |
|
It
has no unit. |
It has
the unit “amu”. |
|
Examples: Na=11,
C=6, O=8, N=7, Cl=17 etc. |
Examples: Na=23 amu, C=12 amu, O=16 amu, N=15 amu,
Cl=35.5 amu etc. |
Students often mix up the following elements. Give the names for each element.
Mg and Mn, K and P, Na and S, Cu and Co.
|
(a) Mg is the symbol of
Magnesium and Mn is the symbol of
Manganese. |
(b) K is the symbol of
Potassium and P is the symbol of
Phosphorus |
(c) Na is the symbol of
Sodium and S is the symbol of
Sulphur |
(d) Cu is the symbol of
copper and Co is the symbol of
Cobalt. |
(a) Classify the following molecules as mono-atomic, diatomic, triatomic, and polyatomic molecules.
|
Monoatomic molecules |
Diatomic molecules |
Triatomic molecules |
Polyatomic molecules |
|
He, Ar |
N2, HCl, |
H2O, CO2 |
S8, H2SO4,
C6H12 O6 |
(b) Classify the following as cation, anion, molecular ion, free radicals, and molecules.
|
Cation |
Anion |
Molecular Ion |
Free radical |
Molecules |
|
Mg+2, Na+ |
O-2, Cl-, |
CH+, CO+, CO3-2,
C2H5O-1 |
CH3 |
CO3, O2, H2O,
Cl2. |
Solutions to Numericals Involving Chemical Calculations
Calculate the number of moles of Butane C4H10 in 151 g of Butane. (at.masses of C=12 amu and H=1 amu).
Given data
No of moles of C4H10 = ? Mass in grams = 151 g
Solution
The formula of No of moles = (Mass in grams) / (Molar mass)…………..(A)
But Molar mass of C4H10 = 4 (at.mass of C) + 10 (at.mass of H) = 4 (12 g) + 10 (1 g)
= 48 g + 10
g
= 58 g
Now by
putting the values in eq (A), we have
No of moles of C4H10 = 151g / 58 g
= 2.63 moles Answer.
What is the mass of 5 moles of ice? (At.masses: H = 1 amu,
O = 16 amu)
Given Data
Mass in gram
of Ice = ?
No of moles
of Ice = 5 moles
Solution
As No of moles= (Mass in
grams)/(Molar mass)________ eq (A)
and Molar mass of H2O = 2 (at.mass
of H) + 1(at.mass of O)
= 2 (1 g) +
1 (16 g)
= 2g + 16g
=18g
Now by
putting the values in eq (A), we have
5 = (Mass
in gram)/18g
By cross
multiplication, we have
Mass in gram
of Ice = 5 x 18
= 90g Answer.
Viii: Calculate the number of molecules in 6.50 mol of CH4
Given: No
of moles= 6.50 mol No of
molecules=?
Solution:
As No of moles= (No of atoms/molecules) / (Avogadro’s Number (NA)
Or 
o of molecules=No of moles x NA………(i)
Putting the
values in eq (i), we have
No of molecules = 6.50
mol x 6.023 x 1023
No of molecules = 39.149
x 1023 molecules
No of molecules = 3.914 x 1024 molecules Ans
Ix: Calculate the average atomic mass of Lithium from the following data.
|
Isotope |
Natural
abundance (%) |
Relative
atomic mass (amu) |
|
6Li |
7.5 |
6.0151 |
|
7Li |
92.5 |
7.0160 |
1St Isotope of Li= 6.0151 %abundance of Li-6= 7.5 %
2nd Isotope of Li=7.0160 %abundance
of Li-7= 92.5 %
Required Average atomic
mass of Lithium?
Solution
The formula for finding
the average atomic mass of an element is given as
= (1st isotope x its %age) + (2nd
isotope x its %age) + ….…) / 100
Putting the values for
Li in the above equation
Average atomic mass of Li = (1st Isotope × its% ) + (2nd Isotope × its%) / 100
= (6.0151 × 7.5) + (7.0160×92.5) / 100
= (45.113+648.98) / 100
= 694.0 / 100
= 6.94 amu Ans
X: Calculate the mass of 6.68 x 1023 molecules of PCl3.
Given: No
of molecules of PCl3 = 6.68
x 1023 molecules of PCl3
Mass in
gram=?
Solution:
As we know that
No of moles = (No of atoms or molecules) / (Avogadro's No (NA) _______ (A)
Putting the values in
eq (A), we have
No of moles = 6.68 x 1023
/ 6.023 x 1023 molecules of
PCl3
No of moles = 1.109
No of moles = Mass in grams / Molar mass
Or Mass in gram = No of moles x Molar mass__________
(B)
Molar mass of PCl3
= 1 (31) + 3 (35.5)
= 31 g + 106.5 g
= 137.5 g
Now putting the values in eq (B)
Mass in gram = 1.109 x
137.5
No of moles = 152.48 moles
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