Atoms And Molecules

The law of conservation of mass states that the mass of the reactants and the product during a chemical equation remains the same
which is mass of the reactants = mass of the product during a chemical reaction.


5.3 g + 6 g = 2.2 +0.9 +8.2
11.3 g = 11.3 g

We find that the mass of reactants is 11.3 g and the mass of products is also 11.3 g. Since the mass of products is equal to the mass of reactants, the given data verify the law of conservation of mass.

Here we have been given that hydrogen and oxygen always combine in the fixed ratio of 1 : 8 by mass. This means that:

1 g of hydrogen gas requires = 8 h of oxygen gas

So, 3 g of hydrogen gas requires = 8 × 3 g of oxygen gas

= 24 g of oxygen gas

Thus, 24 grams of oxygen gas would be required to react completely with 3 grams of hydrogen gas.

The postulate of Dalton’s atomic theory that is the result of the law of conservation of mass are following:

‘Atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction.’

‘The relative number and kinds of atoms are constant in a given compound.’

The postulate of Dalton's atomic theory that explains the law of definite proportions is ’The relative number and kinds of atoms are constant in a given compound.’

One atomic mass unit is a mass unit equal to exactly one twelveth (1/12th) the mass of one atom of carbon-12. The relative atomic masses of all elements have been found with respect to an atom of carbon-12.

It is not possible to see an atom with naked eyes because an atom is a very small particle. For example, the radius of a hydrogen atom is 10^-10 metre.

i) Sodium is an ionic compound made up of sodium ions and oxide ions. The sodium ion has a valency or charge of 1+ where is oxide Ion has a valency or charge of 2-



Na O
1+ 2- (cross over valencies)

Formula: Na₂O

Hence, the formula of sodium oxide is Na₂O.


ii) Aluminium chloride is an ionic compound made up of aluminium ions and chloride ions. Aluminium Ion has a valency of 3+ whereas chloride Ion has a valency of 1-.



Al Cl
3+ 1- (cross over valencies)

Formula: AlCl₃

Hence, the formula of aluminium chloride is AlCl₃.


iii) Sodium sulphide ionic compound made up of sodium ions and sulphide ions. Sodium Ion has a valency of 1+ ion has a valency of 2-



Na S
1+ 2- (cross over valencies)

Formula: Na₂S

Hence, the formula of sodium sulphide is Na₂S.

iv) Magnesium hydroxide is an ionic compound made up of magnesium and Hydroxide ions. Magnesium iron has a valency of 2+ and Hydroxide has a valency of 1-



Mg OH
2+ 1- (cross over valencies)

Formula: Mg(OH)₂

Hence, the formula of magnesium hydroxide is Mg(OH)₂.

(i) The name of this compound is Aluminium sulphate.

(ii) The name of this compound is Calcium chloride.

(iii) The name of this compound is Potassium sulphate.

(iv) The name of this compound is potassium nitrate.

(v) The name of this compound is Calcium carbonate.

The chemical formula of a compound is a symbolic representation of its composition. The combining power or combining capacity of an element is known as its valency. Valency can be used to find out how the atoms of an element will combine with the atom(s) of another element to form a chemical compound. Some elements show more than one valency. Then we must crossover the valencies of the combining atoms. The positive and negative charges must balance each other and the overall structure must be neutral.

(i) There are three atoms in a H₂S molecule. Two hydrogen atoms and one sulphur.

(ii) There are five atoms present in a PO₄^-3 ion. One phosphorus atom and four oxide ions.

The molecular mass of a substance is the sum of the atomic masses of all the atoms in a molecule of the substance. It is therefore the relative mass of a molecule expressed in atomic mass units (u).

(i) Atomic mass of Hydrogen = 1 u.

Molecular mass of H₂ = Mass of 2H atoms = 2 x 1 = 2 u

(ii) Atomic mass of Oxygen = 16 u

Molecular mass of O₂ = Mass of 2 'O' atoms = 2 X 16 = 32 u

(iii) Atomic mass of Cl = 35.5 u

Molecular mass of Cl₂ = Mass of 2Cl atoms = 2 X 35.5 = 71 u

(iv) In this molecule, there is one carbon atom and two oxygen atoms.

Atomic mass of Carbon = 12 u

Atomic mass of Oxygen = 16 u

Molecular mass of C0₂ = Mass of C atom + Mass of 2 '0' atoms

= 12 + 2 X 16 = 12 + 32 = 44 u

(v) Methane molecule has one carbon atom and four hydrogen atoms.

Atomic mass of Carbon = 12 u, Atomic mass of Hydrogen = 1 u

Molecular mass of CH₄ = Mass of C atom + Mass of 4H atoms

= 12 + 4 X 1 = 12 + 4 = 16 u

(vi) C₂H₆ molecule has two carbon atoms and six hydrogen atoms.

Atomic mass of Carbon = 12 u, Atomic mass of Hydrogen = 1 u

Molecular mass of C₂H₆ =Mass of 2C atoms + Mass of 6H atoms

= 2 X 12 + 6 X 1 = 24 + 6 = 30 u

(vii) C₂H₄ molecule has two carbon atoms and four hydrogen atoms.

C₂H₆ molecule has two carbon atoms and four hydrogen atoms.

Molecular mass of C₂H₄ = Mass of 2C atoms + Mass of 4 H atoms

= 2 X 12 + 4 X 1 = 24 + 4 = 28 u

(viii) NH₃ molecule has one nitrogen atom and three hydrogen atoms.

Atomic mass of Nitrogen = 14 , Atomic mass of hydrogen = 1 u

Molecular mass of NH₃ = Mass of N atom + Mass of 3 H atoms

= 14 + 3 X 1 = 14 + 3 = 17 u

(ix) CH₃0H molecule has one carbon atom, four hydrogen atoms and one oxygen atom.

Atomic mass of carbon =n 12 u, Atomic mass of hydrogen = 1 u and Atomic mass of oxygen = 12 u.

Molecular mass of CH₃0H = Mass of C + Mass of 4 H + Mass of 0

= 12 + 4 X 1 + 16 = 12 + 4 + 16 = 32 u

(i) Formula mass of ZnO = Mass of Zn atom + Mass of 0 atom

= 65 + 16 = 81 u

(ii) Formula mass of Na₂0 = Mass of 2Na atoms + Mass of 0

= 2 X 23 + 16 = 46 + 16 = 62 u

(iii) Formula mass of K₂C0₃ = Mass of two potassium atoms + one carbon atom + three oxygen atoms = 2 X39 + 12+ 3 X16 = 78+ 12+ 48 = 138 u

Mass of one mole of carbon atom = 12g

1 mole of a carbon contains 6.022 X 10²³ atoms

Hence, mass of 6.022 X 10²³ Atoms of carbon = 12g

Mass of 1 atom of carbon = 12/6.022 X 10²³ = 1.993 X 10^-23 g

In order to solve this problem, we should convert 100 grams of sodium into moles of sodium, and also 100 grams of iron into moles iron. The element having more moles will have more atoms. Please note that since the atomic mass of sodium is 23u, the molar mass of sodium will be 23 g/mol. Similarly, since the atomic mass of iron is 56u, the molar mass of iron will be 56 g/mol. We will now calculate the moles of sodium atoms (Na) and iron atoms (Fe), one by one.

(i) Moles of sodium = Mass of sodium/Molar mass of sodium

= 100/23

= 4.34

(ii) Moles of iron = Mass of iron/Molar mass of iron

= 100/56

= 1.78

(i) Mass of boron in compound = 0.096 g

And, Mass of compound = 0.24 g

So, Percentage of boron =

=

= 40% (1)

(ii) Mass of oxygen in compound = 0.144 g

And, Mass of compound = 0.24 g

So, Percentage of oxygen =

= 60% (2)

Thus, the percentage composition of the compound is: Boron = 40%; Oxygen = 60%

Answer will be governed by the law of constant proportions which states that the mass of reactant must be equal to the mass of the product formed during a reaction

Now, since carbon and oxygen combine in the fixed proportion of 3 : 8 by mass to produce 11 g of carbon dioxide,


3 + 8 = 11 (by ratio proportion)

According to the question, the equation formed will be:-


3 + 50 = 11 + 42
53 = 53

Therefore, the same mass of carbon dioxide (11 g) will be obtained even if we burn 3 g of carbon in 50 g of oxygen.
The extra oxygen (50 – 8 = 42 g oxygen) will remain unchanged.

Polyatomic ions is the group of atom carrying positive or negative charge.
For example,
ammonium ion, NH₄⁺, is a polyatomic ion which is made up of two types of atoms, nitrogen (N) and hydrogen (H) joined together.
Similarly,
carbonate - CO₃^-2 , sulphate - SO₄^-2, nitrate - NO₃^- and hydroxide ions OH^- are all polyatomic ions.

(a) The chemical formula of a compound is a symbolic representation of its composition. The combining power or combining capacity of an element is known as its valency. Valency can be used to find out how the atoms of an element will combine with the atom(s) of another element to form a chemical compound. Some elements show more than one valency.

While writing the chemical formulae for compounds, we write the constituent elements or their symbols and their valencies as shown below. Then we must crossover the valencies of the combining atoms. The positive and negative charges must balance each other and the overall structure must be neutral.

In magnesium chloride, there will be two chloride ions per one magnesium ion. Thus, the formula or magnesium chloride is MgCl₂

(b) The chemical formula of a compound is a symbolic representation of its composition. The combining power or combining capacity of an element is known as its valency. Valency can be used to find out how the atoms of an element will combine with the atom(s) of another element to form a chemical compound. Some elements show more than one valency.

While writing the chemical formulae for compounds, we write the constituent elements or their symbols and their valencies as shown below. Then we must crossover the valencies of the combining atoms. The positive and negative charges must balance each other and the overall structure must be neutral.

Thus, the formula of Calcium Oxide is CaO.

(c) The chemical formula of a compound is a symbolic representation of its composition. The combining power or combining capacity of an element is known as its valency. Valency can be used to find out how the atoms of an element will combine with the atom(s) of another element to form a chemical compound. Some elements show more than one valency.

While writing the chemical formulae for compounds, we write the constituent elements or their symbols and their valencies as shown below. Then we must crossover the valencies of the combining atoms. The positive and negative charges must balance each other and the overall structure must be neutral.

Thus, the formula or copper nitrate is Cu(NO₃)₂

(d) The chemical formula of a compound is a symbolic representation of its composition. The combining power or combining capacity of an element is known as its valency. Valency can be used to find out how the atoms of an element will combine with the atom(s) of another element to form a chemical compound. Some elements show more than one valency.

While writing the chemical formulae for compounds, we write the constituent elements or their symbols and their valencies as shown below. Then we must crossover the valencies of the combining atoms. The positive and negative charges must balance each other and the overall structure must be neutral.

In Aluminium Chloride, there are three chloride ions per one aluminium ion. Thus, the Aluminium chloride is AlCl_3.

(e) The chemical formula of a compound is a symbolic representation of its composition. The combining power or combining capacity of an element is known as its valency. Valency can be used to find out how the atoms of an element will combine with the atom(s) of another element to form a chemical compound. Some elements show more than one valency.

While writing the chemical formulae for compounds, we write the constituent elements or their symbols and their valencies as shown below. Then we must crossover the valencies of the combining atoms. The positive and negative charges must balance each other and the overall structure must be neutral.

Thus, the formula or calcium carbonate is CaCO₃

(a) Calcium oxide, CaO is also known as quick lime. The elements present in quick lime are: Calcium (Ca) and Oxygen (O).

(b) The chemical formula of hydrogen bromide is HBr. The elements present in hydrogen bromide are: Hydrogen (H) and Bromine (Br).

(c) Baking soda is sodium hydrogen carbonate, and its chemical formula is NaHC0₃. The elements present in baking soda are: Sodium (Na), Hydrogen (H), Carbon (C) and Oxygen (O).

(d) The chemical formula of potassium sulphate is K₂S0₄. The elements present in potassium sulphate are: Potassium (K), Sulphur (S) and Oxygen (O).

The molar masses of all these substances will be equal to the respective molecular masses expressed in g/mol. Now:

(a) Molar mass of ethyne, C₂H₂ = Mass of C × 2 + Mass of H × 2

= 12 × 2 + 1 × 2

= 24 + 2

= 26 g/mol

(b) Molar mass of sulphur molecule, S₈ = Mass of S × 8

= 32 × 8

= 256 g/mol

(c) Molar mass of phosphorus molecule, P₄ = Mass of P × 4

= 31 × 4

= 124 g/mol

(d) Molar mass of hydrochloric acid, HCl = Mass of H + Mass of Cl

= 1 + 35.5

= 36.5 g/mol

(e) Molar mass of nitric acid, HNO₃ = Mass of H + Mass of N + Mass of O × 3

= 1 + 14 + 16 × 3

= 15 + 48

= 63 g/mol

(a) Mass of 1 mole of nitrogen atoms = Atomic mass of nitrogen atom

Atomic mass of nitrogen atom = 14u

Therefore, Mass of 1 mole of nitrogen atoms = 14 g

(b) Mass of 1 mole of Aluminium atoms = Atomic mass of Aluminium

Atomic mass of Aluminium = 27 u

Mass of 1 mole of aluminium atom = 27 g

Therefore, mass of 4 moles of aluminium atoms = 4 X 27 = 108 g

(c) Mass of 1 mole of sodium sulphite = Molecular mass of sodium sulphite (Na₂SO₃) in grams

Molecular mass of Na₂SO₃ in grams = Mass of 2Na + Mass of S +Mass of 3 '0'

= 2 X 23 + 32 + 3 X 16

= 46 + 32 + 48

= 126 g

Therefore, Mass of 10 moles of Na₂SO₃ = 126 x 10 g = 1260 g

(a) Atomic mass of Oxygen = 16 u

Molecular mass of Oxygen (O₂) = 16 x 2 = 32 g

Now, 1 mole of oxygen gas = Molecular mass of oxygen in grams = 32 g

Also, 32 g of Oxygen gas = 1 mole

1 g of Oxygen gas = 1/32 mole

12 g of Oxygen gas = 1/32 X 12 = 0.375 mole

Hence, 12 g of Oxygen gas = 0.375 mole

(b) Molecular mass of water (H₂0) = 1 x 2 + 16 = 18u.

Now, 1 mole of water = Molecular mass of water in grams = 18 g

If 18 g of water = 1 mole

1 g of water = 1/18 mole

Then , 20 g of water = 1/18 X 20 mole = 1.12 moles

Hence, 20 g of water = 1.12 moles

(c) Molecular mass of carbon dioxide (CO₂) = 12 + 16 x 2 = 12 + 32 = 44 u.

Now, 1 mole of carbon dioxide = Molecular mass of carbon dioxide in grams = 44g

If 44g of carbon dioxide = 1 mole

1 g of carbon oxide = 1 /44 mole

Then, 22 g of carbon dioxide =22/44 = 0.5 mole

Hence, 22 g of carbon dioxide = 0.5 mole

(a) The atomic mass of oxygen (O) = 16 u.

The mass of 1 mole of oxygen atoms = 16g.

Now, 1 mole of oxygen atoms = 16g

So, 0.2 mole of oxygen atoms = 16 x 0.2g = 3.2g

Hence, 0.2 mole of oxygen atom = 3.2 g.

(b) The molecular mass of water (H₂0)= 18u,

The mass of 1 mole of water molecules = 18 g

Now, 1 mole of water molecules = 18g

0.5 mole of water molecules = 18 x 0.5g = 9g

Hence, 0.5 mole of water molecules = 9 g

The molecular formula of sulphur is given to be S₈. It contains 8 atoms of sulphur.

The molecular mass of sulphur molecule = 32 x 8 = 256u.

Therefore, 1 mole of sulphur molecules = 256 g

Now, 256g of sulphur = 1 mole of sulphur molecules

Hence, 16g of sulphur = 16/256 mole of sulphur molecules = 0.0625 moles of Sulphur molecules

Also, 1 mole of sulphur molecules = 6.023 x 10²³ molecules

0.0625 mole of sulphur molecules = 6.023 x 10²³ x 0.0625 molecules = 3.764 x 10²² molecules

Therefore, the number of molecules present in 16 g sulphur is 3.76 x 10²² molecule

To solve this problem, we have to find out the mass of aluminum atoms in 0.051 g of aluminum oxide (which will give us the mass of aluminum ions) This can be done as follows:

1 mole of Al₂O₃ = Formula mass of Al₂O₃ in grams

= Mass of Al × 2 + Mass of O × 3

= 27 × 2 + 16 × 3

= 54 + 48

= 102 grams

Now, 1 mole of Al₂O₃ contains 2 moles of Al.

So, Mass of Al in 1 mole of Al₂O₃ = Mass of Al × 2

= 27 × 2

= 54 grams

Now, 102 g aluminum oxide contains = 54 g Al

Therefore, 1 g aluminum oxide contains = 54/102 Al

So, 0.051 g aluminium oxide contains = 54/102 × 0.051 g Al = 0.027 g Al

The atomic mass of aluminum is 27 u. 1 mole of aluminum atoms (or aluminum ions) has a mass of 27 grams, and it contains 6.022 × 10²³ aluminum ions.

Now, 27 g of aluminum has ions = 6.022 × 10²³ (Avogadro number)

So, 1 g of aluminum has ions = 6.022 × 10²³/27

0.027 g Aluminium has ions =6.022 × 10²³/27 × 0.027= 6.022 × 10²⁰

Thus, the number of aluminum ions (Al³⁺) in 0.051 gram of aluminum oxide is 6.022 × 10²⁰.