# Atoms and Molecules Class 9 NCERT Solutions

Atoms and Molecules Class 9 NCERT Solutions – The notes provided on our website for CBSE students are designed to align with CBSE and NCERT syllabus guidelines. By referring to these notes, students can save time and gain a comprehensive understanding of important topics and questions in each chapter. This can help them better retain key concepts and be well-equipped to answer any questions that may appear on exams.

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## Atoms and Molecules Class 9 NCERT Solutions

##### Q. In a reaction, 5.3 g of sodium carbonate reacted with 6 g of acetic acid. The products were 28.2 g of sodium acetate, 2.2 g of carbon dioxide, and 0.9 g of water. Establish the consistency of these data with the law of mass conservation. sodium carbonate + acetic acid → sodium acetate + carbon dioxide + water.

Answer – The law of conservation of mass states that in any chemical reaction, the total mass of the reactants must be equal to the total mass of the products.

Given:

Mass of sodium carbonate = 5.3 g
Mass of acetic acid = 6 g
Mass of carbon dioxide produced = 2.2 g
Mass of water produced = 0.9 g
Mass of sodium acetate produced = 8.2 g

Total mass of reactants = 5.3 g + 6 g = 11.3 g
Total mass of products = 2.2 g + 0.9 g + 8.2 g = 11.3 g

##### Q. Water is created when the mass ratio of hydrogen to oxygen is 1:8. What mass of oxygen gas would be necessary for 3 g of hydrogen gas to totally react?

Answer – According to the given ratio, for every 1 part of hydrogen, 8 parts of oxygen are required to form water.

Therefore, the mass of oxygen required to react completely with 3 g of hydrogen can be calculated as follows:

Mass of oxygen = (8/1) x 3 g of hydrogen
Mass of oxygen = 24 g of oxygen

Therefore, 24 g of oxygen gas would be required to react completely with 3 g of hydrogen gas.

##### Q. Which atomic theory postulate is a direct effect of the law of conservation of mass?

Answer – The concept of conservation of mass leads to the following assumption in Dalton’s atomic theory:

“A chemical process cannot create, destroy, or divide atoms into smaller particles, which are the building blocks of all elements.”

##### Q. What atomic theory postulate best explains the law of definite proportions?

Answer – The second postulate of Dalton’s atomic theory—according to which atoms of various elements combine in compounds in predetermined ratios—is the only one that can adequately explain the rule of definite proportions. This postulate states that the ratio of atoms from various elements that combine to create a compound is a constant for that particular compound, independent of the origin of the elements, and that this ratio is known as the atomic ratio. In other words, the rule of definite proportions results from the fact that the ratio of the masses of the constituent elements in a compound is always the same.

##### Q. Define the atomic mass unit.

Answer – The atomic mass unit (amu) is a unit of mass used for expressing atomic and molecular weights. It is defined as exactly one-twelfth of the mass of one atom of the carbon-12 isotope. The mass of all other elements is measured relative to this standard. Therefore, the relative atomic masses of all elements are determined with respect to an atom of carbon-12, which is assigned a mass of exactly 12 amu.

##### Q. Why can’t we see an atom with our naked eyes?

Answer – It is not possible to see an atom with naked eyes because atoms are extremely small, even compared to the smallest visible objects. The size of an atom is on the order of 10-10 meters, which is much smaller than the wavelength of visible light.

##### Q. Write down the formulae of(i) sodium oxide(ii) aluminium chloride(iii) sodium suphide(iv) magnesium hydroxide

(i) Sodium oxide – Na2O
(ii) Aluminium chloride – AlCl3
(iii) Sodium sulphide – Na2S
(iv) Magnesium hydroxide – Mg(OH)2

##### Q. List the names of the compounds that the following equations represent:(i) Al2(SO4)3(ii) CaCl2(iii) K2SO4(iv) KNO3(v) CaCO3

Below are the names of the compounds for each of the following formulae –

(i) Al2(SO4)3 – Aluminium sulphate
(ii) CaCl2 – Calcium chloride
(iii) K2SO4 – Potassium sulphate
(iv) KNO3 – Potassium nitrate
(v) CaCO3 – Calcium carbonate

##### Q. What does the phrase “chemical formula” mean?

Answer – Chemical formula is a symbolic representation of a chemical compound that consists of the chemical symbols of the elements present in the compound, along with their subscripts to indicate the relative proportions of each element. It provides information about the composition of the compound, such as the type and number of atoms of each element in the compound. The chemical formula helps in identifying the compound and predicting its chemical and physical properties.

##### Q. How many atoms are present in a(i) H2S molecule and(ii) PO4 3– ion?

(i) There are 3 atoms in an H2S molecule – 2 atoms of hydrogen and 1 atom of sulfur.
(ii) There are 5 atoms in a PO4 3- ion – 1 atom of phosphorus and 4 atoms of oxygen.

##### Q. Determine the molecular masses of H2, O2, Cl2, CO2, CH4, C2H6, C2H4, NH3, CH3OH.

Answer – The molecular masses of the given compounds can be calculated using the following equations:

(i) Molecular mass of H2 = 2 × Atomic mass of H
= 2 × 1
= 2 u

(ii) Molecular mass of O2 = 2 × Atomic mass of O
= 2 × 16
= 32 u

(iii) Molecular mass of Cl2 = 2 × Atomic mass of Cl
= 2 × 35.5
= 71 u

(iv) Molecular mass of CO2 = Atomic mass of C + 2 × Atomic mass of O
= 12 + 2 × 16
= 44 u

(v) Molecular mass of CH4 = Atomic mass of C + 4 × Atomic mass of H
= 12 + 4 × 1
= 16 u

(vi) Molecular mass of C2H6 = 2 × Atomic mass of C + 6 × Atomic mass of H
= 2 × 12 + 6 × 1
= 30 u

(vii) Molecular mass of C2H4 = 2 × Atomic mass of C + 4 × Atomic mass of H
= 2 × 12 + 4 × 1
= 28 u

(viii) Molecular mass of NH3 = Atomic mass of N + 3 × Atomic mass of H
= 14 + 3 × 1
= 17 u

(ix) Molecular mass of CH3OH = Atomic mass of C + 4 × Atomic mass of H + Atomic mass of O + Atomic mass of H
= 12 + 4 × 1 + 16 + 1
= 32 u

##### Q. Determine the formula unit masses of ZnO, Na2O and K2CO3, Using the atomic weights of Zn = 65 u, Na = 23 u, K = 39 u, C = 12 u, and O = 16 u.

Answer – The formula unit mass is the sum of the atomic masses of all the atoms in a formula unit of the compound.

The formula of ZnO is ZnO.
Formula unit mass of ZnO = Atomic mass of Zn + Atomic mass of O
= 65 u + 16 u
= 81 u

The formula of Na2O is Na2O.
Formula unit mass of Na2O = 2 × Atomic mass of Na + Atomic mass of O
= 2 × 23 u + 16 u
= 62 u

The formula of K2CO3 is K2CO3.
Formula unit mass of K2CO3 = 2 × Atomic mass of K + Atomic mass of C + 3 × Atomic mass of O
= 2 × 39 u + 12 u + 3 × 16 u
= 138 u

Therefore, the formula unit masses of ZnO, Na2O, K2CO3 are 81 u, 62 u, and 138 u, respectively.

##### Q. What is the mass of one carbon atom in grammes if a mole of carbon atoms weighs 12 grammes?

Answer – The mass of one mole of carbon atoms is 12 grams, which is also known as the molar mass of carbon.

To find the mass of one atom of carbon, we can use Avogadro’s number, which states that there are 6.022 x 1023 atoms in one mole of an element.

So, the mass of 1 atom of carbon is given by:

mass of 1 mole of carbon atoms / Avogadro’s number
= 12 g / 6.022 x 1023 atoms
= 1.99 x 10-23 g

Therefore, the mass of 1 atom of carbon is 1.99 x 10-23 grams.

##### Q. Which contains more atoms, 100 grammes of sodium or 100 grammes of iron (assuming that Na atomic mass is 23 u and Fe is 56 u)?

Answer – To determine which substance has more number of atoms, we need to calculate the number of moles of each substance using their given masses and molar masses, and then compare the number of atoms in each substance.

The molar mass of sodium (Na) is 23 g/mol and the molar mass of iron (Fe) is 56 g/mol.

For 100 grams of sodium:
Number of moles of Na = Mass of Na / Molar mass of Na
Number of moles of Na = 100 g / 23 g/mol
Number of moles of Na = 4.35 moles

Number of atoms of Na = Number of moles of Na × Avogadro’s number
Number of atoms of Na = 4.35 moles × 6.022 × 1023 atoms/mol
Number of atoms of Na = 2.6193 × 1024 atoms

For 100 grams of iron:
Number of moles of Fe = Mass of Fe / Molar mass of Fe
Number of moles of Fe = 100 g / 56 g/mol
Number of moles of Fe = 1.79 moles

Number of atoms of Fe = Number of moles of Fe × Avogadro’s number
Number of atoms of Fe = 1.79 moles × 6.022 × 1023 atoms/mol
Number of atoms of Fe = 1.07938 × 1024 atoms

Therefore, 100 grams of iron has more number of atoms than 100 grams of sodium.

##### Q. Analysis revealed that a 0.24 g sample of an oxygen and boron combination had 0.144 g of oxygen and 0.096 g of boron. Determine the compound’s weight-based % composition.

Mass of the compound = 0.24 g
Mass of boron in the compound = 0.096 g
Mass of oxygen in the compound = 0.144 g

To calculate the percentage composition by weight, we need to find the total mass of all the elements in the compound.

Total mass of the compound = Mass of boron + Mass of oxygen = 0.096 g + 0.144 g = 0.24 g

Now we can calculate the percentage composition of each element by weight:

Percentage composition of boron = (mass of boron in the compound / total mass of the compound) x 100%
= (0.096 g / 0.24 g) x 100%
= 40%

Percentage composition of oxygen = (mass of oxygen in the compound / total mass of the compound) x 100%
= (0.144 g / 0.24 g) x 100%
= 60%

As a result, the compound has a weight percentage of 40% boron and 60% oxygen.

##### Q. When 11.00 g of carbon dioxide is created when 3.0 g of carbon burn in 8.00 g of oxygen. When 3.00 g of carbon is burned in 50.00 g of oxygen, what mass of carbon dioxide results? Which chemical combination law will control your response?

Answer – The law of definite proportions will govern the answer to the question.

To calculate the mass of carbon dioxide formed when 3.00 g of carbon is burnt in 50.00 g of oxygen, we need to first calculate the amount of oxygen required to react with 3.00 g of carbon:

1 mol of carbon reacts with 1 mol of oxygen to form 1 mol of carbon dioxide.

Carbon dioxide has a molar mass of 44 g/mol.
Oxygen has a molar mass of 32 g/mol.

Therefore, 3.00 g of carbon requires (3.00/12) = 0.25 mol of carbon.

This requires 0.25 mol of oxygen.
0.25 mol of oxygen is equal to (0.25 x 32) = 8.0 g of oxygen.
Since we have 50.00 g of oxygen, it is in excess and all of the carbon will react with 8.0 g of the oxygen.
So, 1 mol of carbon reacts with 1 mol of oxygen to form 1 mol of carbon dioxide.

Therefore, 0.25 mol of carbon will react with 0.25 mol of oxygen to form 0.25 mol of carbon dioxide.

The mass of 0.25 mol of carbon dioxide is (0.25 x 44) = 11.0 g.
Therefore, when 3.00 g of carbon is burnt in 50.00 g of oxygen, 11.0 g of carbon dioxide will be formed.

##### Q. What are polyatomic ions? Provide examples.

Answer – Ions made up of two or more atoms that are covalently bound together and have a total electrical charge are referred to as polyatomic ions. These ions play a significant role in many biochemical processes and are frequently used in chemical reactions.

Here are some instances of polyatomic ions:

Ammonium ion (NH4+)
Nitrate ion (NO3-)
Sulfate ion (SO42-)
Carbonate ion (CO32-)
Hydroxide ion (OH-)
Phosphate ion (PO43-)

(a) MgCl2
(b) CaO
(c) Cu(NO3)2
(d) AlCl3
(e) CaCO3

##### Q. Name the elements that make up the following compounds.(a) Quick lime(b) Hydrogen bromide(c) Baking powder(d) Potassium sulphate.

(a) Quick lime – Calcium and Oxygen
(b) Hydrogen bromide – Hydrogen and Bromine
(c) Baking powder – Sodium, Hydrogen, Carbon, and Oxygen
(d) Potassium sulphate – Potassium, Sulphur, and Oxygen

##### Q. The following compounds’ molar masses should be determined.(a) Ethyne, C2H2(b) Sulphur molecule, S8(c) Phosphorus molecule, P4 (Atomic mass of phosphorus = 31)(d) Hydrochloric acid, HCl(e) Nitric acid, HNO3

(a) Molar mass of C2H2 = 2(atomic mass of C) + 2(atomic mass of H) = 2(12.01) + 2(1.01) = 26.04 g/mol
(b) Molar mass of S8 = 8(atomic mass of S) = 8(32.06) = 256.48 g/mol
(c) Molar mass of P4 = 4(atomic mass of P) = 4(31.00) = 124.00 g/mol
(d) Molar mass of HCl = atomic mass of H + atomic mass of Cl = 1.01 + 35.45 = 36.46 g/mol
(e) Molar mass of HNO3 = atomic mass of H + atomic mass of N + 3(atomic mass of O) = 1.01 + 14.01 + 3(16.00) = 63.01 g/mol

##### Q. What is the mass of—(a) 1 mole of nitrogen atoms?(b) 4 moles of aluminium atoms (Atomic mass of aluminium = 27)?(c) 10 moles of sodium sulphite (Na2SO3)?

(a) The atomic mass of nitrogen (N) is 14 u. Therefore, the mass of 1 mole of nitrogen atoms will be 14 g/mol.

(b) The atomic mass of aluminium (Al) is 27 u. Therefore, the mass of 1 mole of aluminium atoms will be 27 g/mol. So, the mass of 4 moles of aluminium atoms will be:

mass = 4 moles × 27 g/mol mass = 108 g

(c) The formula mass of sodium sulphite (Na2SO3) can be calculated as follows:

Na2SO3 = (2 × atomic mass of Na) + atomic mass of S + (3 × atomic mass of O) Na2SO3 = (2 × 23) + 32 + (3 × 16) Na2SO3 = 46 + 32 + 48 Na2SO3 = 126 u/mol

Therefore, the mass of 10 moles of sodium sulphite (Na2SO3) will be:

mass = 10 moles × 126 g/mol mass = 1260 g

##### Q. Convert into mole.(a) 12 g of oxygen gas(b) 20 g of water(c) 22 g of carbon dioxide.

(a) The molar mass of oxygen is 32 g/mol.
Therefore, the number of moles in 12 g of oxygen gas can be calculated as follows:
Number of moles = Mass / Molar mass = 12 g / 32 g/mol = 0.375 mol
Therefore, 12 g of oxygen gas is equal to 0.375 mole.

(b) The molar mass of water is 18 g/mol.
Therefore, the number of moles in 20 g of water can be calculated as follows:
Number of moles = Mass / Molar mass = 20 g / 18 g/mol = 1.11 mol
Therefore, 20 g of water is equal to 1.11 moles.

(c) The molar mass of carbon dioxide is 44 g/mol.
Therefore, the number of moles in 22 g of carbon dioxide can be calculated as follows:
Number of moles = Mass / Molar mass = 22 g / 44 g/mol = 0.5 mol
Therefore, 22 g of carbon dioxide is equal to 0.5 mole.

##### Q. What is the mass of:(a) 0.2 mole of oxygen atoms?(b) 0.5 mole of water molecules?

(a) The molar mass of oxygen is 16 g/mol. Therefore, the mass of 0.2 mole of oxygen atoms is:
0.2 mol x 16 g/mol = 3.2 g
Hence, the mass of 0.2 mole of oxygen atoms is 3.2 g.

(b) The molar mass of water (H2O) is 18 g/mol. Therefore, the mass of 0.5 mole of water molecules is:
0.5 mol x 18 g/mol = 9 g
Hence, the mass of 0.5 mole of water molecules is 9 g.

##### Q. Determine how many sulphur (S8) molecules are contained in 16 g of solid sulphur.

The molar mass of S8 is:

Molar mass of S8 = 8 x Atomic mass of S = 8 x 32 g/mol = 256 g/mol
Now, we can calculate the number of moles of S8 present in 16 g of solid sulphur using the formula:
Number of moles = Mass ÷ Molar mass
Number of moles of S8 = 16 g ÷ 256 g/mol = 0.0625 mol
One mole of S8 contains Avogadro’s number (NA) of molecules. Therefore, the number of molecules of S8 present in 0.0625 mol of S8 is:
Number of molecules of S8 = 0.0625 mol x NA

= 0.0625 mol x 6.022 x 1023/mol
= 3.765 x 1022 molecules

Therefore, there are approximately 3.765 x 1022 molecules of S8 present in 16 g of solid sulphur.