WAJA F4 Chemistry 2010
Chapter 3 : Chemical Formulae and Equations
CHAPTER 3 : CHEMICAL FORMULAE AND EQUATIONS
A
RELATIVE ATOMIC MASS (RAM) AND RELATIVE MOLECULAR MASS (RMM)
Learning Outcomes
You should be able to:

state the meaning of relative atomic mass based on carbon-12 scale,

state the meaning of relative molecular mass based on carbon-12 scale,

state why carbon-12 is used as a standard for determining relative atomic
mass and relative molecular mass,

calculate the relative molecular mass of substances.
Activity 1 (refer text book pg 28 )
Relative atomic mass of an element , Ar
= The average mass of an atom of the element
1/12 x the mass of an atom of carbon-12
Example:
Ar of C=12
Ar of O=16
Ar of Mg=24
1.
The Relative atomic mass of an element is ……………………………………………………………...
………………………………….
2.
when compare with 1/12 of the mass of an atom of carbon – 12.
Carbon-12 is chosen because it is a ………………………. and can be easily handled.
3. Find the relative atomic masses of these elements.
Element
Relative Atomic Mass
Element
Calcium, Ca
Sodium, Na
Iron, Fe
Copper, Cu
Carbon, C
Hydrogen, H
Potassium, K
Lithium, Li
Bromine, Br
Argon, Ar
Silver, Ag
Caesium, Cs
Lead, Pb
Chlorine, Cl
Flourine, F
Aluminium, Al
Zinc, Zn
Helium, He
Activity 2 (refer text book pg 29 )
Relative molecular mass of a substance, Mr
= The Average mass of a molecule of the substance
1/12 x the mass of an atom of carbon-12
1
Relative Atomic Mass
WAJA F4 Chemistry 2010
Chapter 3 : Chemical Formulae and Equations
Calculating Relative molecular mass,Mr
Mr= The sum of Ar of all atoms present in one molecule
2 Hydrogen
atoms
Molecular
formula
Example:
Mr of Water, H2O = 2(1) + 16 = 18
Relative atomic mass
for Oxygen
Relative atomic mass
for Hydrogen
Mr of Carbon dioxide, CO2 = 12 + 2(16) = 44
For ionic substance , Relative formula mass , Fr
= The sum of Ar of all atoms present in the formula
All Ar, Mr and
Fr have no unit
Example:
Fr of Magnesium oxide, MgO = 24 + 16 = 40
Fr of Sodium chloride, NaCl = 23 + 35.5 = 58.5
1. The relative molecular mass of a molecule is ………………………………………………
………………………………………………………. when compared with 1/12 of the mass
of one atom of ……………………………………………
2. Calculate the relative molecular masses of the substances in the table below.
Substance
Molecular formula
Hydrogen gas
Propane
H2
C3H8
Ethanol
Bromine gas
Methane
Glucose
C2H5OH
Br2
CH4
C6H12O6
Relative molecular mass, Mr
2(1) = 2
Ammonia
NH3
[Relative atomic mass : H,1; C,12; O,16; Br,80 ; N,14 ]
3. Calculate the relative formula masses of the following ionic compounds in the table.
Substance
Potassium oxide
Aluminium sulphate
Compound formula
K2O
Relative formula mass, Fr
2(39) + 16 = 94
Al2(SO4)3
2(27)+3[32+4(16)]=342
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WAJA F4 Chemistry 2010
Chapter 3 : Chemical Formulae and Equations
Zinc nitrate
Zn(NO3)2
Aluminium nitrate
Al(NO3)3
Calcium carbonate
CaCO3
Calcium hydroxide
Ca(OH)2
Hydrated copper(II)
sulphate
CuSO4.5H2O
64 + 32 + 4(16) + 5[2(1) + 16]=250
Hydrated sodium
carbonate
Na2CO3.10H2O
Sodium hydrogen
sulphate
NaHSO4
Aluminium chloride
AlCl3
Copper(II) sulphate
CuSO4
Zinc carbonate
ZnCO3
Potassium
carbonate
K2CO3
[Relative atomic mass: O,16; C,12; H,1; K,39 ; Cu,64 ; Zn, 65; Cl, 35.5 ; Al, 27 S,32 ;
Ca, 40; Na,23; N, 14]
B THE MOLE AND THE NUMBER OF PARTICLES
Learning Outcomes
You should be able to:
 define a mole as the amount of matter that contains as many particles as the
number of atoms in 12 g of 12C,
 state the meaning of Avogadro constant,
 relate the number of particles in one mole of a substance with the Avogadro
constant,
 solve numerical problems to convert the number of moles to the number of
particles of a given substance and vice versa.
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WAJA F4 Chemistry 2010
Chapter 3 : Chemical Formulae and Equations
Activity 3 (refer text book pg 30 )
1. To describe the amount of atoms, ions or molecules , mole is used.
2. A mole is an amount of substance that contains as many particles as the ………………..
…………………………………………………………….. in exactly 12g of carbon-12.
3. A mole is an amount of substance which contains a constant number of particles
atoms, ions, molecules which is 6.02 x 1023
4. The number 6.02 x 1023 is called …………………………………… (NA)
5. In other words:

1 mol of atomic substance contains ……………………………. atoms

1 mol of molecular substance contains ……………………………. molecules

1 mol of ionic substance contains ……

…………………………….. formula units
6. Relationship between number of moles and number of particles (atom/ion/molecules):
x Avogadro Constant
number of moles
number of particles
∻ A vogadro Constant
Number of moles
0.5 mol of carbon atoms
0.2 moles of hydrogen gas ( H2)
Number of particles
…………………………………… atoms of carbon
(i)
(ii)
2 mol of carbon dioxide molecules
…………………………..molecules
of hydrogen gas
…………………………….Atoms of hydrogen
………………x 10 23 molecules of carbon dioxide gas
contains :
………………. atoms of C and
…………………. atoms of O
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WAJA F4 Chemistry 2010
Chapter 3 : Chemical Formulae and Equations
0.007 mol of calcium ions
……………………… calcium ions
…………………………. mol of water
6.02 x 10 25 molecules of water
0.4 mol of ozone gas ( O3)
………………….x 10 23 molecules of ozone,
contains :
……………………… atoms of Oxygen.
7. Complete these sentences .
a) 1 mol of calcium contains ………………………………………….. atoms
b) 2 mol of iron contains
……………………………………………….. atoms
c) 2 mol of magnesium oxide, (MgO) contains ………………………………………….. ions
d) 2 mol of sodium carbonate, (Na2CO3) contains ……………………………………….
e) 3 mol of carbon dioxide, (CO2) contains …………………………………….. molecules
f) 0.5 mol Copper (II) nitrate, Cu(NO3)2 contains ………………………………….. Cu2+ ions
and …………………………………………………. NO3- ions
C NUMBER OF MOLES AND MASS OF SUBSTANCES
Learning Outcomes
You should be able to:

state the meaning of molar mass,

relate molar mass to the Avogadro constant,

relate molar mass of a substance to its relative atomic mass or relative molecular mass,

solve numerical problems to convert the number of moles of a given substance to its
mass and vice versa.
Activity 4 (refer text book pg 33 )
1. The molar mass of a substance
= The molar mass of _________________ mole of the substance.
=
The mass of (NA) number of particles
=
The mass of ____________________ particles
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WAJA F4 Chemistry 2010
Chapter 3 : Chemical Formulae and Equations
x Molar mass
Number of moles
Mass in g
∻ Molar mass
2. Calculating the Mass from a number of Moles
Number of moles
=
.
mass of the substance
.
Mass of 1 mole of the substance
Mass of substance
=
Number of moles x Mass of 1 mole
Therefore :
Example 1 :
What is the mass of 2 moles of carbon ?
Mass
=
=
2 x
24g
12
Example 2 : What is the mass of 2 moles of H2O ?
Mass
=
2
=
36g
x
[ 2(1) +
16 ]
3. Calculate the masses of these substances
a) 2 moles of aluminium atoms
Mass =
b) 10 moles of iodine atoms
Mass =
c) 3 moles of lithium atoms
Mass =
d) 0.5 moles of oxygen gas (O2)
Mass =
e) 0.1 moles of sodium
Mass =
f)
g) 1 mole of carbon dioxide ( CO2)
Mass =
h) 3 moles of nitric acid, ( HNO3 )
Mass =
i)
j)
2 moles of calcium carbonate (CaCO3 )
Mass =
6
2 moles of chlorine molecules (Cl2)
Mass =
0.25 moles of calcium chloride (CaCl2 )
Mass =
WAJA F4 Chemistry 2010
Chapter 3 : Chemical Formulae and Equations
k) 0.25 moles of sodium hydroxide (NaOH)
Mass =
l)
0.25 moles of sodium carbonate (Na2CO3)
Mass =
m) 0.5 moles of potassium manganate (VII)
(KMnO4)
Mass =
n) 0.25 moles of hydrated magnesium sulphate
(MgSO4.7H2O)
Mass =
Activity 5
4. Calculate the Number of Moles from a given Mass
Example : How many moles are there in 88g of CO2
Number of moles = 88
44
= 2 moles
a) 2g of helium atoms
Number of moles =
b) 6g of carbon atoms
Number of moles =
c) 16g of helium atoms
Number of moles =
d) 4g of sulphur atoms
Number of moles =
e) 4g of oxygen molecules (O2)
Number of moles =
f) 213g of chlorine molecules (Cl2)
Number of moles =
g) 0.56g of nitrogen molecules (N2)
Number of moles =
h) 254g of iodine molecules (I2)
Number of moles =
i) 88g of carbon dioxide (CO2)
Number of moles =
j) 3.1g of sulphur dioxide (SO2)
Number of moles =
k) 560g of potassium hydroxide (KOH)
Number of moles =
l) 392g of sulphuric acid (H2SO4)
Number of moles =
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WAJA F4 Chemistry 2010
Chapter 3 : Chemical Formulae and Equations
m) 170g of ammonia (NH3)
Number of moles =
n) 120g of magnesium oxide (MgO)
Number of moles =
o) 4g of sodium hydroxide (NaOH)
Number of moles =
p) 73g of hydrogen choride (HCl)
Number of moles =
q) 15.8g of potassium manganate (VII)
KMnO4
Number of moles =
r) 8g of ammonium nitrate (NH4NO3)
Number of moles =
s) 0.78g of aluminium hydroxide Al(OH)3
Number of moles =
t) 0.92g of ethanol (C2H5OH)
Number of moles =
Activity 6
5. Complete the following table.
Element/compound
Chemical
formulae
Molar mass
Copper
Cu
RAM= 64
Calculate
(a)Mass of 1 mol = ……………g
(b) Mass of 2 mol = …………. g
(c)Mass of ½ mol = ………….g
(d)Mass of 3.01x1023 Cu atoms
=
Sodium hydroxide
NaOH
RFM= 40
(a) Mass of 3 mol of sodium hydroxide =
(b) Number of moles of sodium hydroxide in
20 g =
Zinc nitrate
Zn(NO3)2
RFM =
a) Number of moles in 37.8 g of zinc nitrate :
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WAJA F4 Chemistry 2010
D
Chapter 3 : Chemical Formulae and Equations
NUMBER OF MOLES AND VOLUME OF GAS
Learning Outcomes
You should be able to:

state the meaning of molar volume of a gas,

relate molar volume of a gas to the Avogadro constant,

make generalization on the molar volume of a gas at a given temperature and
pressure,

calculate the volume of gases at STP or room conditions from the number of moles
and vice versa,

solve numerical problems involving number of particles, number of moles, mass of
substances and volume of gases at STP or room conditions.
Activity 7 (refer text book pg 36, 37 )
1. The molar volume of a gas is defined as the ………………………………………………….
…………………………………………………………….
2. One mole of any gas always has the …………………………………………… under the same
temperature and pressure.
3. The molar volume of any gas is

24 dm3 at ………………………………………………

22.4 dm3 at …………………………………………….
or
Example :
1 mol of oxygen gas, 1 mol of ammonia gas, 1 mol helium gas and 1 mol sulphur dioxide gas occupies
the same volume of 24 dm3 at room condition
x 22.4 / 24 dm3
Number of moles of gas
Volume of gas
x 22.4/24 dm3
∻22.4/24 dm3
4. Calculate the volume of gas in the following numbers of moles at STP
Example : Find the volume of 1 mole of CO2 gas
Volume
=
=
=
number of moles x
1 x 22.4 dm3
22.4 dm3
9
22.4 dm3
WAJA F4 Chemistry 2010
Chapter 3 : Chemical Formulae and Equations
a) 3 moles of oxygen
Volume =
b) 2 moles of CH4
Volume =
c) 0.3 moles of Argon
Volume =
d) 0.2 moles of SO3
Volume =
e) 0.1 moles of N2
Volume =
f) 1.5 mol of N2
Volume =
5. Complete the diagram below . (Refer to Page 33,34 & 38-Chemistry textbook)
Volume of gas (dm3)
Mass in gram
Number of moles
No of particles
Activity 8
Solve these numerical problems
1.
What is the volume of 0.3 mole of sulphur dioxide gas at STP?
[Molar volume: 22.4 dm3 mol-1 at STP]
(Ans: 6.72 dm3)
2.
Find the number of moles of oxygen gas contained in a sample of 120 cm3 of the gas
at room conditions.
[Molar volume: 24 dm3 mol-1 at room conditions]
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WAJA F4 Chemistry 2010
Chapter 3 : Chemical Formulae and Equations
(Ans: 0.005 mol)
3.
Calculate the number of water molecules in 90 g of water, H2O.
[Relative atomic mass: H, 1; O, 16. Avogadro constant, NA: 6.02 x 1023 mol-1]
(Ans; 3.01x 1024 molecules)
4.
What is the volume of 24 g methane ,CH4 at STP?
[Relative atomic mass: H, 1; C, 12. Molar volume: 22.4 dm3 mol-1 at STP]
(Ans: 33.6 dm3)
5.
How many aluminium ions are there in 20.4 g of aluminium oxide, Al2O3?
[Relative atomic mass: O, 16; Al, 27. Avogadro constant, NA: 6.02 x 1023 mol-
(2 x 0.2 x 6.02 x1023)
6.
Calculate the number of hydrogen molecules contained in 6 dm3 of hydrogen gas at
room conditions.
[Molar volume: 24 dm3 mol-1 at room conditions Avogadro constant, NA: 6.02 x 1023
mol-1]
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WAJA F4 Chemistry 2010
Chapter 3 : Chemical Formulae and Equations
(Ans: 1.505x1023 molecules)
7.
3
23
Find the volume of nitrogen in cm at STP that consists of 2.408 x 10 nitrogen molecules.
[Molar volume: 22.4 dm3 mol-1 at STP. Avogadro constant, NA: 6.02 x 1023 mol-1]
(Ans: 8.96 dm3 )
E CHEMICAL FORMULAE
Learning Outcomes
You should be able to
 state the meaning of chemical formula
 state the meaning of empirical formula
 state the meaning of molecular formula
 determine empirical and molecular formula of substances
 compare and contrast empirical formula with molecular formula
 solve numerical problems involving empirical and molecular formula.
 write ionic formula of ions
 construct chemical formulaf ionic compounds
 state names of chemical compounds using IUPAC nomenclature.
 use symbols and chemical formula for easy and systematic communication in the field
of chemistry.
ACTIVITY 9 (Refer text book pg 40)
1) A Chemical formula - A representation of a chemical substance using letters for
……………………………………… and subscripts to show the numbers of each type of
…………………….. that are present in the substance.
The letter H
shows
…………….
…………….
H2
Subscript shows 2
hidrogen atoms in
a molecule
2) Complete this table
Chemical subtance
Chemical
Notes
formulae
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WAJA F4 Chemistry 2010
Chapter 3 : Chemical Formulae and Equations
……………..
Water
………..
2 atoms of H combine with 1 atom of O
NH3
……. atoms of H combine with 1 atom of N
C3H8
…….. atoms of C combine with ……. atoms of
Propane
H
Magnesium oxide
………………..
……………..
…………………………………………….
H2SO4
……………………………………………
3). There are two types of chemical formulae. Complete the following:
The simplest …………
** Empirical Formula 
……….. ratio of atoms of each ……….
in the compound.
** Molecular Formula 
The actual …………… of atoms of each …………… that are
present in a molecule of the compound
Molecular formula = (Empirical formula)n
Remember:
Example: (i) Compound – Ethene
(ii)
Compound – Glucose
Molecular formula - C6 H12O 6
Molecular formula - C2 H 4
Empirical formula - ...................
Empirical formula - ....................
Activity 10
1 Find the empirical formula of a compound
Example of calculation:
a) When 11.95 g of metal X oxide is reduced by hydrogen, 10.35 g of metal X is
produced. Find the empirical formula of metal X oxide [ RAM; X,207; O,16 ]
Element
X
O
Mass of element(g)
10.35
11.95-10.35
Number of moles of atoms
10.35÷207
(11.95-10.35)÷16
Ratio of moles
Simplest ratio of moles
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WAJA F4 Chemistry 2010
Chapter 3 : Chemical Formulae and Equations
Empirical formula : …………
b) A certain compound contains the following composition:
Na 15.23%, Br 52.98% , O 31.79%, [ RAM : O, 16; Na, 23; Br,80]
(Assume that 100g of substance is used)
Element
Na
Br
O
Mass of element(g)
15.23
52.98
31.79
Number of moles atoms
15.23 ÷23
52.98÷80
31.79÷16
Ratio of moles
Simplest ratio of moles
Empirical formula:: ……………………………………………….
c)
Complete the table below.
Compound
Molecular Formula
Water
H2O
Carbon Dioxide
CO2
Sulphuric Acid
H2SO4
Ethene
C2H4
Benzene
C6H6
Glucose
C6H12O6
Empirical formula
Value of n
CO2
CH2
d) 2.52g of a hydrocarbon contains 2.16 g of carbon. The relative molecular mass of the
hydrocarbon is 84. [RAM H,1; C,12]
i.
Find the empirical formula of the hydrocarbon
ii.
Find the molecular formula of the carbon.
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WAJA F4 Chemistry 2010
Activity 11
Chapter 3 : Chemical Formulae and Equations
:Chemical Formula for ionic compounds:
Complete the table below :
Cation
Formula
Anion
Formula
Hydrogen ion
H
Flouride ion
F
Lithium ion
Chloride ion
Sodium ion
Bromide ion
Potassium ion
Iodide ion
Magnesium ion
Hydroxide ion
Calcium ion
Ca 2
Nitrate ion
Barium ion
Ba 2
Manganate(VII) ion
Copper(II) ion
CH 3COO 
Ethanoate ion
O 2
Iron(II) ion
Iron (III) ion
Sulphate ion
Lead (II) ion
Sulphide ion
Zinc ion
Carbonate ion
Chromium (III) ion
Dichromate (VI) ion
S 2
Cr2O7
Al 3
Aluminium ion
Ammonium ion
PO4
2
3
Chromate (VI) ion
Avtivity 12
a) Chemical formula of an ionic compound comprising of the ions Xm+ and Yn- is constructed
by exchanging the charges of each element. The formula obtained will XnYm
Example : Sodium oxide
Copper (II) nitrate
Na+
O2-
Cu2+
NO3-
+1
-2
+2
-1
2
1
1
2
= Na2O
=
15
....................
WAJA F4 Chemistry 2010
b)
Chapter 3 : Chemical Formulae and Equations
Construct a chemical formula for each of the following ionic compounds:
(i)
Magnesium chloride
(ii)
Potassium carbonate
(iii)
Calcium sulphate
(iv)
Copper (II) oxide
(v)
Silver nitrate
(vi)
Zinc nitrate
(vii) Aluminium oxide
(viii) Iron(II) hydroxide
(ix)
(x)
Lead(II) sulphide
Chromium(III) sulphate
CHEMICAL EQUATIONS
Learning Outcomes
You should be able to
1. state the meaning of chemical equation
identify the reactants and products of a chemical equation
2. write and balance chemical equations
3. interpret chemical equations quantitatively and qualitatively
4. solve numerical problems using chemical equations
5. identify positive scientific attitudes and values practiced by scientist in doing research
6. justify the need to practice positive scientific attitudes and good values in doing researsh
7. use chemical equations for easy and systematic communication in the field of chemistry.
Activity 13 (refer text book pg 48)
Example:
C (s) + O2 (g)  CO2 (g)
Reactant
product
1) Qualitative aspect of chemical equation:
a) Arrow in the equation  the way the reaction is occurring
b) Substances on the left-hand side 
……………………..
c) Substances on the right-hand side  ………………………
d) State of each substance  ………: (s), ………………(l), gas ……….and aqueous solution
……………….
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WAJA F4 Chemistry 2010
2)
Chapter 3 : Chemical Formulae and Equations
Quantitative aspect of chemical equations
Coefficients in a balanced equation  the exact proportions of reactants and products in
equation.
Example:
2 H 2 (g)
+ O2 (g)

2 H 2O (l)
(Interpreting): 2 molecules (2 mol) of H 2 react with 1 molecule (1 mol) of O2 to produced 2 molecules(2
mol) of water
Complete the following word equations and write in chemical equation
a) Sodium
…………
b)
…………………………..
chlorine 
+
+ …………… 
NaCl
Carbon
+
………..

Carbon dioxide
……….
+
…………

……………………..
+
oxygen

……………………………
+
………..

…………………………..
c) Sulphur
………..
d) Zinc
…………
+ oxygen

………………………………..
+ O2

………………………………..
3) Write a balanced equation for each of the following reactions and interpret the equations
quantitatively.
(a). Carbon monoxide gas + oxygen gas  carbon dioxide gas
………………………………………………………………………………………………………
Interpreting:
……………………………………………………………………………………………………………
(b). Hydrogen gas + nitrogen gas  ammonia gas
……………………………………………………………………………………………………….
Interpreting:
…………………………………………………………………………………………………………..
(c). Aluminium + Iron (III) oxide  aluminium oxide + Iron
……………………………………………………………………………………………………….
Interpreting:
…………………………………………………………………………………………………………….
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WAJA F4 Chemistry 2010
Chapter 3 : Chemical Formulae and Equations
Activity 14
** Numerical Problems Involving Chemical Equations
Hydrogen peroxide decomposes according to the following equation:
2 H 2O2 (l)  2 H 2O (l) + O2 (g)
1). Calculate the volume of oxygen gas, O2 measured at STP that can be obtained from the
decomposition of 34 g of hydrogen peroxide, H 2O2 .
[Relative atomic mass : H, 1 ; O, 16. Molar volume : 22.4 dm 3 mol 1 at STP]
(Ans: 11.2 dm3)
2).Silver carbonate Ag2CO3 breaks down easily when heated to produce silver metal
2 Ag2CO3(l)
4 Ag (s) + 2 CO2 (g) + O2
Find the mass of silver carbonate that is required to produce 10 g of silver
[Relative atomic mass: C, 12 ; O, 16 ; Ag, 108]
(Ans : 12.77g)
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WAJA F4 Chemistry 2010
Chapter 3 : Chemical Formulae and Equations
3). 16 g of copper (II) oxide, CuO is reacted with excess methane, CH 4 . Using the equation below, find
the mass of copper that is produced.
[Relative atomic mass : Cu, 64 ; O, 16]
4 CuO (s) + CH 4 (g)  4 Cu (s) + CO2 (g) + 2 H 2O (l)
(Ans : 12.8 g)
4). A student heats 20 g of calcium carbonate CaCO3 strongly. It decomposes according to the
equation below:
CaCO3 (s)
CaO (s) + CO2 (g).
(a). If the carbon dioxide produced is collected at room conditions, what is its volume?
(b). Calculate the mass of calcium oxide, CaO produced.
[Relative atomic mass: C, 12 ; O, 16; Ca, 40. Molar volume :
24 dm3 mol 1 at room conditions]
(Ans : (a). 4.8 dm
19
3
(b) 11.2 g)