Answer the questions.[Che-2M]

Question 12 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 22 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 32 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 42 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 52 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 62 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 72 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 82 Marks

What is the significance of the formula of a substance?

Answer

Significance of formula of a substance:

Formula represents the name of the substance.
Formula represents one molecule of a substance.
Formula gives the number of atoms of each element present in one molecule.
Formula also represents one mole of molecules of the substance.

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Question 92 Marks

What is the significance of the formula H₂O?

Answer

Significance of the formula H₂O:

H₂O represents water.
It represents one molecule of water.
H₂O also represents 6.022 × 10²³ molecules of water.
It represents 18gm of water.

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Question 102 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 112 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 122 Marks

What is the difference between the molecule of an element and the molecule of a compound? Give one example of each.

Answer

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.

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 132 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 142 Marks

What is the atomicity of the following?

Oxygen
Ozone
Neon
Sulphur
Phosphorus
Sodium

Answer

Oxygen = 2
Ozone = 3
Neon = 1
Sulphur = 8
Phosphorus = 4
Sodium = 1

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Question 152 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 162 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 172 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/ 12^thof the mass of a 126CC612atom.

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Question 182 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 192 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 202 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 212 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 222 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 232 Marks

What do the following denote?

N
2N
N₂
2N₂

Answer

N– One atom of nitrogen.
2N– Two separate atoms of nitrogen.
N₂– One molecule of nitrogen.
2N₂– Two molecules of nitrogen.

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Question 242 Marks

What do the following abbreviation stand for?

O
2O
O₂
3O₂

Answer

O^- One atom of oxygen.
2O^- Two separate atoms of oxygen.
O_2^-One molecule of oxygen.
3O_2^-Three molecules of oxygen.

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Question 252 Marks

What are (i) ionic compounds, and (ii) molecular compounds? Give two examples of each type of compounds.

Answer

Lonic compounds: The compounds which are formed by combination of metals and non-metals are called ionic compounds. For ex. CaCl₂ and CaCO₃
Molecular compounds: These compounds are formed by the combination between two non-metal elements. For ex. HCl and H₂S

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Question 262 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:

Sodium phosphate.
Ammonium sulphate.
Calcium hydroxide.
Lead bromide.

Answer

Sodium phosphate: Na₃PO₄
Ammonium sulphate: (NH4)₂SO₄
Calcium Hydroxide: Ca(OH)₂
Lead bromide: PbBr₂

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Question 272 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:

Sodium sulphide.
Copper nitrate.

Answer

Na₂S
Cu(NO₃)₂

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Question 282 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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Question 292 Marks

Name any two laws of chemical combination.

Answer

Two laws of chemical combination are:

Law of conservation of mass.
Law of multiple proportions.

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Question 302 Marks

Is it possible to see atoms these days? Explain your answer.

Answer

Yes, ‘THE SCANNING TUNNELLING MICROSCOPE’ enables people to see atoms. This microscope can produce computer generated images of the surface of elements which show the individual atoms. The atoms show up as blurred images.

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Question 312 Marks

If sulphur exists as S₈ molecules, calculate the number of moles in 100g of sulphur.
(S = 32u)

Answer

Given mass of sulphur = 100g
Molar mass of S₈ = 32 × 8 g = 256g
No. of moles $=\frac{\text{Given mass}}{\text{Molar mass}}$
$=\frac{100}{256}=0.39\text{ moles}$

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Question 322 Marks

If one mole of nitrogen molecules weights 28g, calculate the mass of one molecule of nitrogen in grams.

Answer

6.022 × 10²³ molecules of N₂ weigh = 28gm
So, 1 molecule of N₂ will weigh $=\frac{28}{6.022\ \times\ 10^{23}}\text{ grams of N}_2$
$=4.648\times10^{23}\text{grams of N}_2$

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Question 332 Marks

If 6g of oxygen contains 1 mole of oxygen atoms, calculate the mass of one atom of oxygen.

Answer

6.022 × 10²³ atoms of Oxygen weigh = 16g
So, mass of 1 atom of Oxygen $=\frac{16}{6.022\ \times\ 10^{23}}=2.656\times10^{-23}\text{g}$

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Question 342 Marks

‘If 100 grams of pure water taken from different sources is decomposed by passing electricity, 11 grams of hydrogen and 89 grams of oxygen are always obtained’. Which chemical law is illustrated by this statement?

Answer

According to the given information, water has been taken from different sources and still decomposes into same amount of oxygen and hydrogen, therefore, this is an example of the law of constant proportions.

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Question 352 Marks

‘If 100 grams of calcium carbonate (whether in the form of marble or chalk) are decomposed completely, then 56 grams of calcium oxide and 44 grams of carbon dioxide are obtained’. Which law of chemical combination is illustrated by this statement?

Answer

If 100 grams of calcium carbonate is decomposed in any form (marble or chalk), 56 grams of calcium oxide and 44 grams of carbon dioxide are obtained. This reaction illustrates the law of conservation of mass.

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Question 362 Marks

How many moles of O₂ are there in 1.20 × 10²² oxygen molecules?

Answer

One mole of O₂ contains = 6.022 × 10²³ molecules of oxygen
So, 1 molecule of O₂ has $=\frac{1}{6.022\ \times\ 10^{23}}\text{ moles of O}_2 $
Therefore, 1.02 × 10²³ molecules of O₂ will have $=\frac{1.2\ \times\ 10^{22}}{6.022\ \times\ 10^{23}}\text{ moles of O}_2$
$=0.0199\text{ moles of O}_2$

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Question 372 Marks

How many moles of calcium carbonate (CaCO₃) are present in 10g of the substance?
(Ca = 40u; C = 12u; O = 16u)

Answer

Give mass of CaCO₃ = 10g
Molar mass of CaCO₃ = 1 × Ca + 1 × C + 3 × O
= (40 + 12 + 48)gm = 100gm
So, no. of moles of CaCO₃ $=\frac{\text{Given mass}}{\text{Molar mass}}=\frac{10}{100}=0.1\text{ moles}$

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Question 382 Marks

How many moles are there in 34.5g of sodium?
(Atomic mass of Na = 23u)

Answer

1 mole of sodium weighs = 23gm
So, 1gm of sodium will have $=\frac{1}{23}\text{ moles of sodium}$
Therefore, 34.5gm of sodium will have $=\frac{34.5}{23}=1.5\text{ moles of sodium}$

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Question 392 Marks

How many moles are represented by 100g of glucose, C₆H₁₂O₆?
(C = 12u, H = 1u, O = 16u)

Answer

Molar mass of glucose = 180g
180g of glucose has = 1 mol
So, 100g of glucose will have $=\frac{1\ \times\ 100}{180}=0.55\text{ moles}$

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Question 402 Marks

How many atoms are there in 0.25 mole of hydrogen?

Answer

1 mole of hydrogen contains = 6.022 × 10²³ atoms of hydrogen
So, 0.25 moles of hydrogen will have = 6.022 × 10²³ × 0.25 = 1.50 × 10²³atoms of hydrogen.

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Question 412 Marks

Explain the significance of the symbol H.

Answer

Significance of symbol H:

It represents Hydrogen element.
It represents one atom of Hydrogen element.
It represents one mole of Hydrogen atoms.
It represents 2gms of Hydrogen.

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Question 422 Marks

Explain the formation of (i) sodium ion, and (ii) chloride ion, form their respective atoms giving the number of protons and number of electrons in each one of them. What is the reason for positive charge on a sodium ion and a negative charge on a chloride ion?

Answer

$\text{Na (11 protons, 11 electrons)}\xrightarrow{\ \ \ \ \ {-1\text{ electron}}\ \ \ \ \ }\text{Na}^+\text{(11 protons, 10 electrons)}$

The reason for positive charge on sodium is the loss of electron.

$\text{Cl (17 protons, 17 electrons)}\xrightarrow{\ \ \ \ \ {+1\text{ electron}}\ \ \ \ \ }\text{Cl}^-\text{(17 protons, 18 electrons)}$

The reason for negative charge on chlorine is the gain of electron.

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Question 432 Marks

Define mole. What are the two things that a mole represents?

Answer

A group of 6.022 × 10²³ particles (atoms, molecules or ions) of a substance is called a mole of that substance. One mole represents the amount of a substance equal to its 'GRAM ATOMIC MASS' or 'GRAM MOLECULAR MASS' and 6.022 × 10²³no. of particles of the substance.

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Question 442 Marks

Define ‘molecular mass’ of a substance.

Answer

The molecular mass of a substance is the relative mass of its one molecule as compared with a 126CC612atom. The mass of an atom of 126CC612 is considered to be 12a.m.u.

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Question 452 Marks

Define gram molecular mass of a substance. How much is the gram molecular mass of oxygen?

Answer

The amount of substance whose mass in grams is numerically equal to its molecular mass is called gram molecular mass of that substance. Gram molecular mass of the oxygen is 32g.

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Question 462 Marks

Define ‘formula unit’ of an ionic compound. What is the formula unit of:

Sodium chloride
Magnesium chloride?

Answer

The simplest combination of ions that produces an electrically neutral unit, is called 'formula unit' of the ionic compound.

Formula unit of sodium chloride: NaCl
Formula unit of magnesium chloride: MgCl₂

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Question 472 Marks

Calculate the number of moles in 12.044 × 10²⁵ atoms of phosphorus.

Answer

6.022 × 10²³ atoms of phosphorus contains = 1 mole of phosphorus
So, 12.044 × 10²⁵ atoms of phosphorus will have $=\frac{12.044\ \times\ 10^{25}}{6.022\ \times\ 10^{23} }=200\text{ moles}$

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Question 482 Marks

Calculate the mole ratio of 240g calcium and 240g of magnesium.
(Ca = 40u; Mg = 24u)

Answer

240g of calcium has $=\frac{240}{40}=6\text{ moles}$
240g of magnesium has $=\frac{240}{24}=10\text{ moles}$
So, required mole ratio = 6 : 10 = 3 : 5

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Question 492 Marks

Calculate the molecular masses of the following compounds:

Methanol, CH₃OH
Ethanol, C₂H₅OH

Answer

Molecular mass of Methanol (CH₃OH) = 1 × C + 3 × H + 1 × O + 1 × H

= (12 + 3 + 16 + 1)u = 32u

Molecular mass of Ethanol (C₂H₅OH) = 2 × C + 5 × H + 1 × O + 1 × H

= (24 + 5 + 16 + 1) = 46u

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Question 502 Marks

Calculate the molecular masses of the following compounds:

Hydrogen sulphide, H₂S
Carbon disulphide, CS₂

Answer

Molecular mass of hydrogen sulphide (H₂S)

= 2 × H + 1 × S

= (2 + 32)u

= 34u

Molecular mass of Carbon disulphide (CS₂)

= 1 × C + 2 × S

= (12 + 2 × 32)u

= 76u

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