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ATOMS AND MOLECULES
Arvind Kumar
Question 1: In a reaction, 5.3 g of sodium carbonate reacted with 6 g of ethanoic acid.
The products were 2.2 g of carbon dioxide, 0.9 g water and 8.2 g of sodium ethanoate. Show
that these observations are in agreement with the law of conservation of mass. Sodium
carbonate + ethanoic acid → sodium ethanoate + carbon dioxide + water
Answer: In the given reaction, sodium carbonate reacts with ethanoic acid to produce
sodium ethanoate, carbon dioxide, and water.
Question 2: Hydrogen and oxygen combine in the ratio of 1:8 by mass to form water. What
mass of oxygen gas would be required to react completely with 3 g of hydrogen gas?
Answer: It is given that the ratio of hydrogen and oxygen by mass to form water is
1:8. Then, the mass of oxygen gas required to react completely with 1 g of hydrogen gas is 8
g. Therefore, the mass of oxygen gas required to react completely with 3 g of hydrogen gas is
8 × 3 g = 24 g.
Question 3: Which postulate of Dalton’s atomic theory is the result of the law of conservation
of mass?
Answer: The postulate of Dalton‟s atomic theory which is a result of the law of
conservation of mass is: Atoms are indivisible particles, which can neither be created nor
destroyed in a chemical reaction.
Question 4: Which postulate of Dalton’s atomic theory can explain the law of
definite proportions?
Answer: The postulate of Dalton‟s atomic theory which can explain the law of
definite proportion is:
The relative number and kind of atoms in a given compound remains constant.
Question 1: Define atomic mass unit.
Answer:
Mass unit equal to exactly one-twelfth the mass of one atom of carbon-12 is called one
atomic mass unit. It is written as „u‟.
Question 2: Why is it not possible to see an atom with naked eyes?
Answer: The size of an atom is so small that it is not possible to see it with naked eyes.
Also, the atom of an element does not exist independently.
Answer: The chemical formula of a compound means the symbolic representation of
the composition of a compound. From the chemical formula of a compound, we can
know the number and kinds of atoms of different elements that constitute the
compound. For example, from the chemical formula CO2 of carbon dioxide, we come to
know that one carbon atom and two oxygen atoms are chemically bonded together to
form one molecule of the compound, carbon dioxide.
Question 4:
How many atoms are present in a
(i) H2S molecule and
(ii) PO4 3− ion?
Answer:
(i) In an H2S molecule, three atoms are present; two of hydrogen and one of sulphur.
(ii) In a PO4 3− ion, five atoms are present; one of phosphorus and four of oxygen.
Question: State law of conservation of mass.
Answer: The mass can neither be created nor destroyed in a chemical reaction.
Question State law of constant proportion.
Answer:A pure chemical compound always consists of the same elements that are
combined together in a fixed proportion by mass.
Question Define atomic mass unit.
Answer: The mass of one twelfth (1/12) of the mass of one atom of carbon taken as 12.
Question What are molecules?
Answer: Molecules represents a group of two or more atoms(same or different) chemically
bonded to each other and held tightly by strong attractive forces. Molecules are of two
types
(a) Molecules of elements
(b) Molecules of compound
Question What do you mean by molecular mass?
Answer: The average relative mass of the molecule as compared to the mass of carbon
taken as 1u.
Question Write the relationship between number of moles and atomic mass.
Answer: Number of moles=given mass/gram atomic mass
Question Why are chemical reactions balanced?
Answer: In all chemical reactions, there is only exchange of atoms of reactants taking
place when products are formed. Since there is no loss or gain of mass ,the chemical
reactions are balanced according to law of conservation of mass.
Question What is basic difference between atoms and molecules?
Answer:Atoms except those of noble or inert gas elements cannot exist of their own.
whereas all molecules can have independent existence. Molecules are made up of atoms.
Question The atomic mass of an element is in fraction .What does it mean?
Answer:If the atomic mass of an element is in fraction, this mean that it exists in the form
of isotopes. The atomic mass is the average atomic mass and is generally fractional.
Question What is the difference between the mass of molecule and molecular mass?
Answer:Mass of a molecule is that of a single molecule also known as its actual mass. But
molecular mass is the mass of Avogadro‟s number of molecules.
Question Where do we use the words mole and mol?
Answer: In the text part we use the word mole It is collection of 6.022x10
while using it as a unit it is written as mol.
22
particles . But
Question How many moles are present in 11.5 g of sodium?
Answer: Gram atomic mass of Na =23g
11.5g of Na represents= (1mol)*(11.5g)/23.5g)=0.5 mol
Question Explain why the number of atoms in one mole of hydrogen gas is double the
number of atoms in one mole of helium gas?
Answer: Hydrogen gas is a diatomic in nature(H2) while helium gas is monoatomic (He).As
a result, the number of atoms in one mole of hydrogen(2*NA )are expected to be double as
compared to number of atoms in one mole of helium(NA)
Question An element Z forms an oxide with formula Z 2O3. What is its valency?
Answer: Since valency is exchanged while writing the formula . Therefore valency of
Z=3
Question The valency of an element A is 4 . Write the formula of its oxide.
Answer: The formula of its oxide is A2O4 or AO2.
Question An element X has valency 3 while the element Y has valency 2.Write the formula of
the compound between X and Y .
Answer: X2Y3
Question : What are ions ?
Answer: Ions are charged atoms or group of atoms. These of two types
(a) cations - positively charged
(b) anions - negatively charged
Question Which postulate of Dalton’s Atomic theory is the basis of law of conservation of
mass?
Answer: “ Atoms can neither be created nor destroyed during a physical or a chemical
change”
Question Write the formulae of sodium oxide and aluminium chloride
Ans Sodium oxide = Na2O
Aluminium chloride =AlCl3
Question Find out the ratio by mass of the combining elements in the following compounds
(a) MgCO3
(b) CH3OH
(c) CaCl2
Answer:
(a) MgCO3
(b) CH3OH
(c) CaCl2
--------------------
Mg :C:O
C:H:O
Ca :Cl
= 24:12:48 = 2:1:4
=12:4:16 =3:1:4
= 40:71
Question 3 : What are polyatomic ions? Give its examples.
Question Define atomicity of the molecule.
Answer: Atomicity of the molecule is the number of atoms present in the molecule.
Question What is the atomicity of oxygen,ozone,neon and sulphur?
Answer: Oxygen O2 = 2
Ozone O3 = 3
Neon Ne =1
S8 =8
Sulphur
Question What is wrong with the statement '1 mol of hydrogen'
Answer: The statement is not correct. We must always write whether hydrogen is in atomic
form or molecular form. The correct statement is :
1mole of hydrogen atoms or
one mole of hydrogen molecules .
Question The atomic mass of an element is in fraction. What does it mean?
Answer: If the atomic mass of an element is in fraction, this mean that it exists in the form
of isotopes. The atomic mass is the average atomic mass and is generally fractional.
Question Write postulates of Dalton’s Atomic Theory.
Answer:
(i) All matter is made of very tiny particles called atoms.
(ii) Atoms are indivisible particles, which cannot be created or destroyed in a chemical
reaction.
(iii) Atoms of a given element are identical in mass and chemical properties.
(iv) Atoms of different elements have different masses and chemical properties.
(v) Atoms combine in the ratio of small whole numbers to form compounds.
(vi) The relative number and kinds of atoms are constant in a given compound.
Question 4 : Give name of the elements present in the following compounds
(a) Quick lime (b) Hydrogen bromide (c) Baking powder (d) Potassium sulphate
Therefore the mass of 0.5 mole of water molecules = 0.5 x 18 g = 9 g