Unit 4: Stoichiometry — Long Questions

Ans. Representing a chemical change is called chemical equation. Chemical equation tell us element or compound which are reacting and those which are being produced as a result of chemical change. It is compulsory to write reactant on left hand side and products on right hand side. An arrow is drawn from reactants to product separate the two.

The following points must be kept in mind while writing a chemical equation.

i. A chemical equation must obey the law of conservation of mass. This means that no atom should be destroyed or produced during a chemical change. The total number and the type of atoms must remain the same during a chemical change.

ii. The formulas of elements and compounds must be written correctly.

iii. A chemical equation must determine the correct mole ratio between the reactants and the products.

iv. A chemical equation must also point out the direction in which the change is proceeding.

v. It is a usual practice to show the normal physical states of reactants and products. Solid, liquid and gas are symbolized as s, ℓ and g respectively. Aqueous (aq) represents the solvated ion.

Ans. Avogadro's Number

The number of particles in one mole of a substance is called Avogadro's number. The value of this number is 6.02×10²³. It is represented as N_A. An Italian scientist Amaedo Avogadro discovered Avogadro's number.

Explanation

In chemistry we deal with substances which are composed of atoms, molecules or formula units. The counting of these particles is not possible for the chemist. The concept of Avogadro's number facilitated the counting of particles contained in the given mass of a substance. Avogadro's Number is a collection of 6.02×10²³ particles. It is represented by symbol N_A. Hence, the 6.02 x 10²³ number of atoms, molecules or formula units are called Avogadro's number that is equivalent to one 'mole' of respective substance. In simple words 6.02 ×10²³ particles are equal to one mole as twelve eggs are equal to one dozen. Following examples show the relationship between the Avogadro's number and the mole of a substance.

Examples

6.02 × 10²³ atoms of carbon are equivalent to one mole of carbon.
6.02 × 10²³ molecules of H₂O are equivalent to one mole of water.
6.02 ×10²³ formula units of NaCl are equivalent to one mole of sodium chloride.

Thus, 6.02×10²³ atoms of elements or 6.02×10²³ molecules of molecular compounds or 6.02×10²³ formula units of ionic compounds are equivalent to 1 mole.

For further explanation about number of atoms in molecular compounds or number of ions in ionic compounds, following two examples are given:

One molecule of water is made up of 2 atoms of hydrogen and 1 atom of oxygen, hence 2 x 6.02×10²³ atoms of hydrogen and 6.02 × 10²³ atoms of oxygen constitute one mole of water. So, total no of atoms in 1 mole of water are 18.06 ×10²³ or 1.806 × 10²⁴.

One formula unit of sodium chloride consists of one sodium ion and one chloride ion. So, there are 6.02×10²³ number of Na⁺ ions and 6.02 × 10²³ Cl⁻ ions in one mole of sodium chloride.

Thus, the total number of ions in 1 mole of NaCl is 12.04×10²³ or 1.204×10²⁴.

Mole (Chemist secret unit)

A mole is defined as the amount (mass) of a substance that contains 6.02×10²³ number of particles (atoms, molecules or formula units). It is abbreviated as 'mol'.

It establishes a link between mass of a substance and number of particles.

Explanation

We know that a substance may be an element or a compound (molecular or ionic). Mass of a substance is either one of the following: atomic mass, molecular mass or formula mass. These masses are expressed in atomic mass units (amu). But when these masses are expressed in grams, they are called as molar masses.

Scientists have agreed that Avogadro's number of particles are present in one molar mass of a substance.

Quantitative definition of mole

The atomic mass, molecular mass or formula mass of a substance expressed in grams is called mole.

Examples

Atomic mass of carbon expressed as 12g =1 mole of carbon
Molecular mass of H₂O expressed as 18g =1 mole of water
Molecular mass of H₂SO₄ expressed as 98g =1 mole of H₂SO₄
Formula mass of NaCl expressed as 58.5g =1 mole of NaCl

Relationship between Mole and Mass:

No. of moles = Known mass of substance(g) / Molar masss of the substance (g / mol)

Mass of substance (g) = number of moles x molar mass
Number of particles = number of moles x 6.02 × 10²³

Ans. Data

mass of CH₄ = 10g

Molar Mass of CH₄= C + 4H
= 12 + 4 (1)
= 12 + 4 = 16g/mol

Molar mass of CO₂ = C + 2O
12 = C + 2 (16)
= 12 + 32 = 44 g/mol

To find

Mass of CO₂= ?

Solution

Firstly, we have to find no of moles of methane, then we will find no of moles of CO₂ through balance Chemical equation

Moles of CH₄ = Mass / Molar mass
= 10 / 16
= 0.6 mol

So, as per balance chemical equation
CH₄ + 2O₂ ——→ CO₂ + 2H₂O
: CO₂
CH₄
0.6 : 0.6

So, 0.6 moles of CO₂ will be produced as per equation.

Mass of CO₂ = moles x Molar mass
= 0.6 × 44
= 27.43 g

27.43g of CO₂ will be produced

Ans Solution:

To find no. of moles of coal needed, we have to find molar ratio between coal and carbon monoxide through balance chemical equation.

3C(s) + O₂(g) + H₂O(l) ——→ H₂ + 3CO
: CO
C
3 : 3
1 : 1
10 : 10

So, 10 moles of Coal will be required to prepare 10 moles of CO.

Ans Data:

Moles of Sulphur = 10

Molar mass of SO₂ = S + 2(O)
= 32 + 2 (16)
= 32 + 32
= 64g/mol

To Find

Mass of SO₂=?

Solution

Firstly, we will find moles of SO₂ through molar ratio between reactant and product through balance chemical equation

2H₂S + SO₂ ——→ 2H₂O + 3S
SO₂ : S
1 : 3
⅓ : 1
10 × ⅓ : 10
3.33

So, no. of moles by SO₂ = 3.33 mol

Mass of SO₂ = Moles x molar mass
= 3.33 x 64
= 213.12g

Result

213.12g of SO₂ will be required.

Ans Data:

Mass of urea = 1kg = 1000g

Molar mass of urea = (NH₂)₂ CO
=2N+ 4H + C + O
=2 (14) + 4(1) + 12 +16
= 28 + 4 + 12 + 16
= 60g/mol

Molar mass of NH₃ = N + 3H
= 14 + 3 (1)
= 14 + 3
= 17g/mol

To Find

Mass of ammonia =?

Solution

Firstly, we will find number of moles of urea, then we will find moles of ammonia through molar ratio given in balance chemical equation

Moles of urea = Mass / Molar Mass
= 1000 / 60
= 16.66

To find no. of moles of ammonia, molar ratio will be compared through balance chemical equation

2NH₃ + CO₂ ——→ NH₂COONH₂ + H₂O
NH₃ : NH₂COONH₂
2 : 1
2 x 16.66 : 16.66
NH₃ : NH₂COONH₂
33.32 : 16.66

So, no. of moles of NH₃ = 33.32 mol

Mass of NH₃ = Moles x Molar Mass
= 33.32 x 17
= 567.77g

Result

567.77 g of ammonia will be required.

(a) Data:

Mass of H₂ = 3g

Molar mass of H₂ = 2H
= 2 (1.008)
= 2.016 g/mol

To Find

no. of atoms = ?

Solution

Firstly no. of moles will be calculated as:

Moles = Mass / Molar mass
= 3 / 2.016
= 1.48 mol

Now, no. of molecules of H₂ will be calculated as:
no. of molecules = moles x N_A
= 1.48 x 6.02 ×10²³
= 8.91 × 10²³

One molecule composed of two Hydrogen atoms
So, Total no of atoms = 2 × 8.91 x 10²³
= 1.78 x 10²⁴

(b) Data:

Moles of N₂ = 3.4mol

To find

No of atoms = ?

Solution

Firstly calculate no. of molecules

no. of molecules = moles x N_A
=3.4 x 6.02 × 10²³
= 2.05 × 10²⁴

One nitrogen molecules composed of two nitrogen atom, So
Total no. of atoms = 2 × 2.05 × 10²⁴
= 4.10 × 10²⁴

(c) Data:

Mass of C₆H₁₂O₆ = 10 g

Molar mass of glucose = C₆H₁₂O₆
= 6 (12) + 12(1) + 6(16)
= 72 + 12 + 96
= 180g/mol

To Find

no. of atoms = ?

Solution

Firstly we will find no. of moles

no. of moles = Mass / Molar mass
= 10 / 180
= 0.055

Now calculate no. of molecules

no. of molecules = moles x N_A
= 0.055 x 6.02 × 10²³
= 3.34 × 10²²

One glucose molecule composed of 6 carbon, 12 Hydrogen and 6 oxygen.
So, it becomes 24 atoms.

no. of atoms = 24 × 3.34× 10²²
= 8.01 × 10²³

Ans To calculate the number of moles of water needed for a single adult who drinks eight glasses of water a day, we can follow these steps:

If one glass occupies 400 cm³, then for eight glasses:
Total volume = 8 glasses × 400 cm³/glass
= 3200 cm³

Since 1 liter = 1000 cm³
Total volume in liters = 3200 cm³ / 1000 = 3.2 liters.

The molar mass of water (H₂O) is approximately 18 g/mol. The density of water is about 1 g/cm³, which means that 1 liter of water, has a mass of about 1000 grams.

Therefore, the number of moles in 3.2 liters of water can be calculated as follows:
Mass of water = 3.2 liters × 1000 g/liter
= 3200 grams.

Number of moles = mass (g) / molar mass (g/mol)
= 3200 g / 18 g/mol
= 177.78 moles.

Therefore, a single adult would need approximately 177.78 moles of water if they drink eight glasses of water every day.

Ans In the chemical formula of sand, SiO₂, represents silicon dioxide, which is the primary component of most types of sand. However, unlike water (H₂O), which consists of discrete molecules, sand does not exist as individual SiO₂ molecules. Instead, sand is made up of a vast network of silicon and oxygen atoms that are bonded together in a solid structure.

Covalent Network Structure In sand, each silicon atom is covalently bonded to four oxygen atoms in a tetrahedral arrangement. Each oxygen atom is, in turn, bonded to two silicon atoms. This creates a three-dimensional network of SiO₂ units that extends throughout the material.

Macroscopic Representation The formula SiO₂ is a way to represent this network structure on a macroscopic scale. It indicates the ratio of silicon to oxygen in the compound, which is 1:2. This formula is useful for understanding the composition of sand, even though it does not reflect the discrete molecular structure like in covalent compounds.

Crystallinity While some forms of sand can be crystalline, such as quartz, others can be amorphous. In both cases, the SiO₂ formula still applies because it describes the overall composition of the material, regardless of its structural arrangement.

Ans. In order to write down the formula of an ionic compound, first identify the cations and anions and the number of charges present on them. Finally combine the two ions together to form an electrically neutral compound.

Steps

If you know the name of binary ionic compound, you can write its chemical formula.

i. Start by writing symbol of cation with its charge.

ii. Then write the symbol of anion with its charge and find out how many of these ions are needed to give an electrically neutral compound.

For example, write down the formula of lithium oxide. The symbol of lithium cation with its single positive charge is Li⁺. The symbol of anion is O²⁻.

iii. Apply crisscross method to write the formula. In this method, the numerical value of each of the ion charges is crossed over to become the subscript of the other ion. Signs of the charges are then dropped.

Example 2

Write down the formula of Aluminium oxide.

Cation Al³⁺ Anion O²⁻ Al₂O₃

Example 3

Write down the formula of Magnesium nitride.

Cation Mg²⁺ Anion N³⁻ Mg₃N₂

Ans.

Solution Mass of CaCO₃ = 25g Molar mass of CaCO₃ = 100 g mol⁻¹
Mass of CaCl₂ product = ? Molar mass of CaCl₂ = 111 g mol⁻¹

According to the equation
100 g of limestone to produce calcium chloride = 111g
1 g of limestone will react to produce calcium chloride = 111/100 g
25g of limestone will react to produce calcium chloride = 111/100 × 25
= 27.75g

Ans. Solution no. of moles of ethyl alcohol = 1.80
no. of moles of oxygen needed = ?

The balanced chemical equation for the reaction will be

C₂H₅OH₍ₗ₎ + 3O₂₍g₎ ——→ 2CO₂₍g₎ + 3H₂O₍l₎

According to this equation
One mole of ethyl alcohol, when completely burnt, needs oxygen = 3 moles
1.8 moles of ethyl alcohol, upon burning, will need oxygen = 3/1 × 1.8
= 5.4 moles

1 mole of ethyl alcohol produces moles of CO₂ = 2.0
1.8 mole of ethyl alcohol will produce = 2/1 × 1.8
= 3.6 moles

Ans. Solution Moles of Al = 0.3 moles Grams of O₂ used = ?

The balanced chemical equation for the reaction.
4Al (s) + 3O₂₍g₎ ——→ 2A₂O₃₍s₎

According to this equation
4 moles of aluminium need oxygen = 3.0 moles
1.0 mole of aluminium will need oxygen = 3/4
0.3 moles of aluminium will need oxygen = 3/4 × 0.3
= 0.225 moles

1 mole of oxygen (O₂) has molar mass = 32g
0.225 mole of oxygen (O₂) will have molar mass = 32 × .0225
= 7.2g

Ans. Solution Mass of H₂ used = 5g
Molecules of water produced = ?

The balanced chemical equation for the reaction:
2H₂₍g₎ + O₂₍g₎ ——→ 2H₂O₍l₎
2 moles 1 mole 2 moles
4g 32 g 36 g

According to this equation
4 g of hydrogen produce H₂O = 36g of H₂O
5 g of hydrogen will produce H₂O = 36/4 × 5 = 45g of H₂O

36g (2 moles) of H₂O contain molecules = 6.02 × 10²³ × 2
= 12.04 × 10²³

45g of H₂O contain = 45/36 × 12.04 × 10²³
= 1.505 × 10²⁴ molecules