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Science Answer the questions.[Che-2M]

Atoms and Molecules · CBSE STD 9 (CBSE - English Medium). 99 questions.

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Question 12 Marks
Write the cations and anions present (if any) in the following compounds.
  1. CH3COONa
  2. NaCI
  3. H2
  4. NH4N03
Answer
S.No.
Compounds
Cation
Anion
(a)
CH3COONa
Na+
CH3COO-
(b)
NaCI
Na+
CI-
(c)
H2
-
-
(d)
NH4N03
$\text{NH}^+_4$
N03
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Question 22 Marks
Write the atomicity of the following:
I2
Answer
I2:The Atomicity of I2 is 2 because it contains two atoms in its molecule.
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Question 32 Marks
With the help of example, explain the law of conservation of mass.
Answer
The law of conservation of mass states that, mass is neither created nor destroyed during chemical reaction.
For example:
The burning of carbon 
carbon + oxygen = carbon dioxide 
According to the law, the mass of carbon and oxygen i.e the mass of reactants is equal to the mass of product carbon dioxide after the reaction.
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Question 42 Marks
Why the symbols of few elements, like sodium, do not start with the initial letter of the name?
Answer
In many cases they do start with the initial letter if you’re speaking a Romance language.
Sodium, specifically, has the symbol Na from the Latin word “natrium” for “soda”. 
Sir Humphrey Davy discovered potassium, named K after “Kalium”.
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Question 52 Marks
Why is it not possible to see an atom even with the most powerful microscope?
Answer
Atoms are the building blocks of nature. They are tiniest particles in nature and optical microscopes cannot magnify the image of such a tiny object, so that becomes visible to us. It is possible to watch an atom under an electron microscope.
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Question 62 Marks
Why definition of an atom given by Dalton is no longer valid?
Answer
The statement of Dalton's theory that atoms are indivisible is not valid since now, under special circumstances, atoms can be further divided into still smaller particles called electrons, protons and neutrons.
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Question 72 Marks
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.
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Question 82 Marks
Which postulate of Dalton’s atomic theory is the result of law of conservation of mass given by Lavoisier?
Answer
The postulate of Dalton's atomic theory, which is the result of the law of conservation of mass proposed by Lavoisier, states that, "Atoms can neither be created nor destroyed."
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Question 92 Marks
Which postulate of Dalton’s atomic theory can explain the law of constant proportions?
Answer
The postulate "The elements consist of atoms having fixed mass, and that the number and kind of atoms of each element in a given compound is fixed" can be used to explain the law of constant proportions.
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Question 102 Marks
Which postulate of Dalton’s atomic theory can explain the law of conservation of mass?
Answer
The postulate "The elements consists of atoms and that atoms can neither be created nor destroyed" can be used to explain the law of conservation of mass.
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Question 112 Marks
Which part of the Dalton’s atomic theory came from the law of constant proportions given by Proust?
Answer
The postulate of Dalton's atomic theory which came from the law of constant proportions proposed by Proust, states that "The 'number' and 'kind' of atoms in a given compound is fixed."
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Question 122 Marks
Which one is a larger quantity molar mass or molecular mass? Explain.
Answer
Molar mass is the mass of 1 mole of molecules of that element.
For example: Molar mass of hydrogen is 1g, This means that 1 mole of hydrogen atoms will have mass around 1g.
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Question 132 Marks
Which law states, that in a chemical compound, elements always combine in a fixed proportion?
Answer
In chemistry, the law of definite proportion, sometimes called Proust's law or the law of definite composition, or law of constant composition states that a given chemical compound always contains its component elements in fixed ratio (by mass) and does not depend on its source and method of preparation.
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Question 142 Marks
Which ancient Indian philosopher suggested that all matter is composed of very small particles? What name was given by him to these particles?
Answer
Maharishi Kanada was the ancient Indian philosopher who suggested that matter comprises of very small particles. He named these particles ‘parmanu’.
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Question 152 Marks
When 3.0g of carbon is burnt in 8.00g oxygen, 11.00g of carbon dioxide is produced. What mass of carbon dioxide will be formed when 3.00g of carbon is burnt in 50.00g of oxygen? Which law of chemical combination will govern your answer?
Answer
Carbon + Oxygen → Carbon dioxide
3g of carbon reacts with 8g of oxygen to produce 11g of carbon dioxide. If 3g of carbon is burnt in 50g of oxygen, then 3g of carbon will react with 8g of oxygen. The remaining 42g of oxygen will be left un-reactive. In this case also, only 11g of carbon dioxide will be formed. The above answer is governed by the law of constant proportions
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Question 162 Marks
What weight of each element is present in 1.5 moles of sodium sulphite, Na2SO3?
(Atomic masses: Na = 23u; S = 32u; O = 16u)
Answer
1.5 moles of Na2SO3 has 3 moles of Na, 1.5 mole of S and 4.5 moles of O.
Thus, mass of sodium = 3 × 23 g = 69g
Mass of sulphur = 1.5 × 32 = 48g
Mass of oxygen = 4.5 × 16 g = 72g
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Question 172 Marks
What is the significance of the formula of a substance?
Answer
Significance of formula of a substance:
  1. Formula represents the name of the substance.
  2. Formula represents one molecule of a substance.
  3. Formula gives the number of atoms of each element present in one molecule.
  4. Formula also represents one mole of molecules of the substance.
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Question 182 Marks
What is the significance of the formula H2O?
Answer
Significance of the formula H2O:
  1. H2O represents water.
  2. It represents one molecule of water.
  3. H2O also represents 6.022 × 1023 molecules of water.
  4. It represents 18gm of water.
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Question 192 Marks
What is the role of valency in the combination of atoms?
Answer
Valency is the combining power of an element. Elements in the same group of the periodic table have the samevalency. The valency of an element is related to how many electrons are in the outer shell.
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Question 202 Marks
What is the number of molecules in 1.5 moles of ammonia?
Answer
One mole of ammonia contains = 6.022 × 1023 molecules of ammonia.
So, 1.5 moles of ammonia contains = 1.5 × 6.022 × 1023
= 9.033 × 1023 molecules of ammonia.
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Question 212 Marks
What is the mass of 5 moles of sodium carbonate (Na2CO3)?
(Atomic masses: Na = 23u; C = 12u; O = 16u)
Answer
1 mole of Na2CO3 = 106g
So, 5 × mole of Na2CO3
= 5 × 106g
= 530g
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Question 222 Marks
What is the fraction of the mass of water due to neutrons?
Answer
Mass of one mole (Avogadro number) of neutrons ~ 1g
Mass of one neutron $=\frac{1}{\text{Avogadro Number (N)}_\text{A}}\text{g}$
Mass of one molecule of water $=\frac{\text{molar mass}}{\text{N}_\text{A}}=\frac{18}{\text{N}_\text{A}}\text{g}$
There are 8 neutrons in one atom of oxygen,
Mass of one neutrons $=\frac{8}{\text{N}_\text{A}}$
Fraction of mass of water due to netrons $\sim\frac{8}{18}$
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Question 232 Marks
What is the difference between the molecule of an element and the molecule of a compound? Give one example of each.
Answer
  1. The molecule of an element contains two or more similar atoms chemically bonded together.

For example: A molecule of hydrogen element consists of 2 hydrogen atoms combined together.

  1. Whereas the molecule of compound contains two or more different type of atoms chemically bonded together.

For example: The molecule of hydrogen chloride (HCl) contains two different type of atoms, i.e. H and Cl.

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Question 242 Marks
What is the difference between a cation and an anion? Explain with examples.
Answer
  • A cation is formed by the loss of one or more electrons by an atom. For ex. Magnesium loses 2 electron to form Mg+2
  • An anion is formed by the gain of one or more electrons by an atom. For Ex. Chlorine loses one electron to form Cl-
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Question 252 Marks
What is the atomicity of the following?
  1. Oxygen
  2. Ozone
  3. Neon
  4. Sulphur
  5. Phosphorus
  6. Sodium
Answer
  1. Oxygen = 2
  2. Ozone = 3
  3. Neon = 1
  4. Sulphur = 8
  5. Phosphorus = 4
  6. Sodium = 1
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Question 262 Marks
What is meant by the symbol of an element? Explain with examples.
Answer
The symbol of element is the “first letter” or “first letter and another letter” of the English name or Latin name of the element.
For example: symbol of Hydrogen is “H” and symbol of Calcium is “Ca”.
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Question 272 Marks
What is meant by the ‘molar mass’ of a substance? State the unit in which molar mass is usually expressed.
Answer
The molar mass of the substance is the mass of 1 mole of that substance. Molar mass is generally expressed in grams or 'g'.
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Question 282 Marks
What is meant by saying that ‘the molecular mass of oxygen is 32’?
Answer
The statement, "the molecular mass of oxygen is 32", means that a molecule of oxygen is 32 times heavier than 1/ 12th of the mass of a 126CC612atom.
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Question 292 Marks
What is meant by atomicity? Explain with two examples.
Answer
The number of atoms present in one molecule of an element is called atomicity of that element.
For example: Atomicity of sodium is 1 and that of nitrogen is 2.
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Question 302 Marks
What is meant by a chemical formula? Write the formulae of one element and one compound.
Answer
A chemical formula represents the composition of a molecule of the substance in terms of the symbols of the elements present in the molecule. It is also known as molecular formula.
  • Chemical formula of element– H2 for hydrogen.
  • Chemical formula of compound– H2O for water.
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Question 312 Marks
What is an ion? How is an ion formed? Explain with the help of two examples of different ions.
Answer
An ion is a positively or negatively charged atom (or group of atoms). An ion is formed by the loss or gain of an electrons by an atom, so it contains an unequal number of protons and electrons.
Example:
  • Sodium ion, Na+, formed by loss of one electron.
  • Chloride ion, Cl-2, formed by gain of one electron.
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Question 322 Marks
What is an atom? How do atoms usually exist?
Answer
An atom is the smallest particle of an element that can take part in a chemical reaction. They usually exist in combination with the atoms of same element or another element.
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Question 332 Marks
What is a molecule? Explain with an example.
Answer
A molecule is an electrically neutral group of two or more atoms chemically bonded together.
For example: Ozone gas has three oxygen atoms combined together, so ozone exists in the form of O3 molecule.
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Question 342 Marks
What do the following denote?
  1. N
  2. 2N
  3. N2
  4. 2N2
Answer
  1. N– One atom of nitrogen.
  2. 2N– Two separate atoms of nitrogen.
  3. N2– One molecule of nitrogen.
  4. 2N2– Two molecules of nitrogen.
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Question 352 Marks
What do the following abbreviation stand for?
  1. O
  2. 2O
  3. O2
  4. 3O2
Answer
  1. O- One atom of oxygen.
  2. 2O- Two separate atoms of oxygen.
  3. O2- One molecule of oxygen.
  4. 3O2- Three molecules of oxygen.
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Question 362 Marks
What are polyatomic ions? Give examples.
Answer
A polyatomic ion is a group of atoms carrying a charge (positive or negative). For example, ammonium ion $\big(\text{NH}_{4}^{+}\big),$ hydroxide ion $\big(\text{OH}^-\big),$ carbonate ion $\big(\text{CO}_{3}^{2-}\big),$ sulphate ion $\big(\text{SO}_{4}^{2-}\big).$
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Question 372 Marks
What are (i) ionic compounds, and (ii) molecular compounds? Give two examples of each type of compounds.
Answer

  1. Lonic compounds: The compounds which are formed by combination of metals and non-metals are called ionic compounds. For ex. CaCl2 and CaCO3

  2. Molecular compounds: These compounds are formed by the combination between two non-metal elements. For ex. HCl and H2S

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Question 382 Marks
Verify by calculating that:
5 moles of CO2 and 5 moles of H2O do not have the same mass.
Answer
Molar mass of CO2 = 44g mol-1
Molar mass of 5 moles of CO2 = 5 × 44g = 220g
Molar mass of H2O = 18g mol-1
Molar mass of 5 moles of H2O = 5 × 18g = 90g
5 moles of CO2 have a mass of 220g and 5 moles of H2O have a mass of 90g.
Hence, they do not have the same mass.
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Question 392 Marks
Verify by calculating that:
240g of calcium and 240g magnesium elements have a mole ratio of 3 : 5.
Answer
240g of calcium and 240g magnesium elements have a mole ratio of 3 : 5.
Number of moles in 240g Ca metal,
$=\frac{240}{40}=6$
Number of moles in 240g Mg metal,
$=\frac{240}{24}=10$
$\therefore$ Mole Ratio = 6 : 10 = 3 : 5
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Question 402 Marks
The valencies (or charges) of some of the ions are given below:
Ion
Valency(Charges)
Ion
Valency(Charge)
Sodium ion
1+
Bromide ion
1-
Ammonium ion
1+
Hydroxide ion
1-
Calcium ion
2+
Sulphate ion
2-
Lead ion
2+
Phosphate ion
3-
Using this information, write down the formulate of the following compounds:
  1. Sodium phosphate.
  2. Ammonium sulphate.
  3. Calcium hydroxide.
  4. Lead bromide.
Answer

  1. Sodium phosphate: Na3PO4

  2. Ammonium sulphate: (NH4)2SO4

  3. Calcium Hydroxide: Ca(OH)2

  4. Lead bromide: PbBr2

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Question 412 Marks
The valencies (or charges) of some of the ions are given below:
Ion
Valency(Charges)
Ion
Valency(Charge)
Sodium ion
1+
Nitrate ion
1-
Copper ion
2+
Sulphide ion
2-
Using this information, write down the formulae of:
  1. Sodium sulphide.
  2. Copper nitrate.
Answer
  1. Na2S
  2. Cu(NO3)2
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Question 422 Marks
The mass of one steel screw is 4.11g. Find the mass of one mole of these steel screws. Compare this value with the mass of the earth (5.98 × 1024kg). Which one of the two is heavier and by how many times?
Answer
One mole of screws weigh = 4.11 × (6.022 × 1023) = 2.475 × 1024g = 2.475 × 1021kg

$\frac{\text{mass of the Earth}}{\text{mole of 1 mole of screws}}=\frac{5.98\times10^{24}\text{kg}}{2.475\times10^{21}\text{kg}}=2.4\times10^3$

The mass of the Earth is 2.4 × 103 times the mass of one mole of steel screws.

$\therefore$ The earth is 2400 times heavier than one mole of steel screws.

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Question 432 Marks
The difference in the mass of 100 moles each of sodium atoms and sodium
ions is 5.48002g. Compute the mass of an electron.
Answer
Mass of 100 mole of electrons = 5.48002g

$\therefore$ Mass of 1 mole of electron

$=\frac{5.48002}{100}\text{g}$

$\therefore$ Mass of 1 electron

$=\frac{5.48002}{100\times6.022\times10^{23}}=9.1\times10^{-28}\text{g}=9.1\times10^{-31}\text{kg}$

 

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Question 442 Marks
Predict the total number of atoms in case:
Sodium oxide.
Answer
Sodium oxide (Na2O):
There are two sodium atoms and one oxygen atom.
Total number of atoms = 2Na + 1O
Hence, there are total 3 atoms.
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Question 452 Marks
Predict the total number of atoms in case:
Magnesium bromide.
Answer
Magnesium bromide (MgBr2):
There are two bromine atoms and one magnesium atom.
Total number of atoms = 2Br + 1Mg
Hence, there are total 3 atoms.
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Question 462 Marks
Predict the total number of atoms in case:
Hydrogen sulphide.
Answer
Hydrogen sulphide (H2S):
There are two hydrogen atoms and one sulphur atom.
Total number of atoms = 2H + 1S
Hence, there are total 3 atoms.
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Question 472 Marks
Predict the total number of atoms in case:
Calcium phosphate.
Answer
Calcium phosphate [Ca3(PO4)2]:
There are three calcium atoms, two phosphorus atoms and eight oxygen atoms.
Total number of atoms = 2Ca + 2P + 8O
Hence, there are total 12 atoms.
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Question 482 Marks
Predict the total number of atoms in case:
Aluminium hydroxide.
Answer
Aluminium hydroxide [Al(OH)3]:
There are three hydrogen and oxygen atoms and one atom of aluminum.
Total number of atoms = 3H + 3O + 1Al
Hence, there are total 7 atoms.
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Question 492 Marks
Predict the mass possessed by certain number of oxygen atoms which is equal to the number of molecules present 1g of hydrogen gas?
Answer
Mass of oxygen atoms = number of molecules present 1g of hydrogen gas
Number of molecules in $1\text{g} \ \text {H}_2=\frac{1}{2\ }\text{mole}$
$\Rightarrow\frac{1}{2}\times6.022\times10^{23}\text{atoms}$
$\Rightarrow3.01\times10^{23}$
Thus, the mass possessed by certain number of oxygen atoms is $3.01\times10^{23}$
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Question 502 Marks
Name the elements water is made of. What are the valencies of these elements? Work out the chemical formula for water.
Answer
Water is made up of Hydrogen and oxygen.
Valency of hydrogen is +1;
Valency of oxygen is -2.
Chemical formula of water is H2O.
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