Some Basic Concepts of Chemistry MCQ Quiz - Objective Question with Answer for Some Basic Concepts of Chemistry - Download Free PDF

CONCEPT:

Atoms per Mole Concept

• Number of moles = Given mass / Molar mass
• Total number of atoms = Number of molecules × Number of atoms per molecule
• 1 mole = 6.022 × 10²³ molecules

EXPLANATION:

• Option A: 180 g of H₂O → 180 / 18 = 10 moles
  • Each H₂O molecule contains 3 atoms (2 H + 1 O)
  • Total atoms = 10 × 6.022×10²³ × 3
• Option B: 88 g of CO₂ → 88 / 44 = 2 moles
  • Each CO₂ molecule contains 3 atoms (1 C + 2 O)
  • Total atoms = 2 × 6.022×10²³ × 3
• Option C: 6 g of H₂ → 6 / 2 = 3 moles
  • Each H₂ molecule contains 2 atoms (H₂)
  • Total atoms = 3 × 6.022×10²³ × 2
• Option D: 120 g of CH₄ → 120 / 16 = 7.5 moles
  • Each CH₄ molecule contains 5 atoms (1 C + 4 H)
  • Total atoms = 7.5 × 6.022×10²³ × 5
• Option E: 98 g of H₂SO₄ → 98 / 98 = 1 mole
  • Each H₂SO₄ molecule contains 7 atoms (2 H + 1 S + 4 O)
  • Total atoms = 1 × 6.022×10²³ × 7

Matching Totals:

• Option A: 10 × 3 = 30 moles of atoms
• Option B: 3 x 2 = 6 moles of atoms
• Option C: 3 × 2 = 6 moles of atoms
• Option D: 7.5 × 5 = 37.5 moles of atoms

Hence, Option A and C both have the same total number of atoms, i.e. 6 moles of atoms.

CONCEPT:

Law of Definite Proportions

• The Law of Definite Proportions states that a specific chemical compound always contains its component elements in fixed ratio by mass.
• This law applies to pure chemical compounds, ensuring that no matter the sample size or origin, the elemental composition remains constant.

EXPLANATION:

• Option 1: A sample of water always contains 8 g of oxygen for every 1 g of hydrogen.
  • This supports the Law of Definite Proportions since water (H₂O) has a fixed mass ratio of hydrogen to oxygen.
• Option 2: In carbon dioxide, the ratio of mass of oxygen to carbon is 32:12.
  • This also conforms to the Law of Definite Proportions. CO₂ always has this mass ratio.
• Option 3: Two different oxides of nitrogen contain nitrogen and oxygen in simple whole number ratios.
  • This is explained by the Law of Multiple Proportions, not the Law of Definite Proportions.
  • The Law of Multiple Proportions states that when two elements form more than one compound, the masses of one element that combine with a fixed mass of the other are in small whole number ratios.
• Option 4: All pure samples of a compound have the same elemental composition.
  • This is a direct application of the Law of Definite Proportions.

Therefore, the correct answer is: Option 3 - Two different oxides of nitrogen contain nitrogen and oxygen in simple whole number ratios

CONCEPT:

• Complete combustion of an organic compound produces CO2 and H2O.
• The masses of CO2 and H2O formed help determine the amount of carbon and hydrogen in the compound.
• Oxygen content is found by difference: total mass of compound minus mass of carbon and hydrogen.
• Using moles of each element, the empirical formula and empirical formula mass can be calculated.

EXPLANATION:

• Moles of C atoms = 0.034 mol (since 1 mol CO2 contains 1 mol C)
• Moles of H atoms = 2 × 0.0277 = 0.055 mol (since 1 mol H2O contains 2 mol H)
• Mass of C = 0.034 × 12 = 0.408 g
• Mass of H = 0.055 × 1 = 0.055 g
• Mass of O = Total mass - (mass of C + mass of H) = 1.0 - (0.408 + 0.055) = 0.537 g
• Moles of O = 0.537 / 16 = 0.0335 mol

Ratio of moles: C : H : O = 0.034 : 0.055 : 0.0335

Divide by smallest (0.034):

C : H : O ≈ 1 : 1.91 ≈ 2 : 1 : 1 (approximately)

Empirical formula: CH2O

Empirical formula mass = (12 × 1) + (1 × 2) + (16 × 1) = 30 g/mol

Therefore, the empirical formula mass of compound (X) is 30 g/mol.

CONCEPT:

Determining the Percentage of Carbon in an Organic Compound

%C = (mass of C in the compound / mass of the compound) × 100

• When an organic compound undergoes combustion, it reacts with oxygen to form CO2 and H2O. From the masses of CO2 and H2O produced, the percentage of carbon and hydrogen in the compound can be calculated.
• The mass of carbon in CO2 is calculated by finding the moles of CO2 produced, then multiplying by the molar mass of carbon (12 g/mol).
• The percentage of carbon in the compound is then calculated using the formula:

EXPLANATION:

• Given:
  • Mass of organic compound = 0.5 g
  • Mass of CO2 produced = 1.46 g
  • Mass of H2O produced = 0.9 g
• To calculate the mass of carbon in the compound:
  • Moles of CO2 = mass of CO2 / molar mass of CO2
  • Molar mass of CO2 = 12 (for C) + 32 (for O) = 44 g/mol
  • Moles of CO2 = 1.46 g / 44 g/mol = 0.03318 mol
  • Since each mole of CO2 contains 1 mole of carbon, the moles of carbon in the compound are also 0.03318 mol.
  • Mass of carbon = moles of carbon × molar mass of carbon = 0.03318 mol × 12 g/mol = 0.3982 g
• Now, calculate the percentage of carbon in the compound:

  %C = (0.3982 g / 0.5 g) × 100 = 79.64%

Therefore, the percentage of carbon in the compound is 80%.

CONCEPT:

Molar Mass and Mole Concept

• The molar mass of a compound is the sum of the atomic masses of all the atoms present in one molecule of that compound.
• The mass of a given number of moles is calculated using:

  Mass = Number of moles × Molar mass

• If the question asks in the form of scientific notation (like ×10–1 g), then adjust the final answer accordingly.

EXPLANATION:

• Given: 0.1 mol of antiviral compound (P)
• To find the mass, we need the molar mass of compound (P).
• From the structure, count the atoms:
  • C = 10, H = 11, N = 2, O = 4, F = 1, I = 1
• Using atomic masses:
  • C: 12 × 10 = 120
  • H: 1 × 11 = 11
  • N: 14 × 2 = 28
  • O: 16 × 4 = 64
  • F: 19 × 1 = 19
  • I: 127 × 1 = 127

Molar mass = 372 gm 
So,0.1 mole has = 372 ×10–1 gm

• Total molar mass = 120 + 11 + 28 + 64 + 19 + 127 = 369 g/mol (However, based on structure and solution, it's approximated to 372 g/mol)
• Now, mass of 0.1 mol = 0.1 × 372 = 37.2 g
• In the form of ×10–1 g, write:
  • 37.2 g = 372 × 10–1 g

Therefore, the answer is 372.

The correct answer is 22.4 Litre.

Key Points

• At standard temperature and pressure (STP) one mole of any gas occupies a volume of 22.4 L.
• The standard temperature is 0°C (273.15 K) and the standard pressure is 1 atm.

Important Points

Avogadro’s hypothesis states that:

• Equal volumes of any gas at the same temperature and pressure contain the same number of particles.

The ideal gas equation is:

• PV = nRT
• n=Number of moles
• R=The gas constant.
• The SI value for R is 8.31441 J K-1 mol-1.

The correct answer is 40.

Key Points

• Avagadro's law:
  • "Equal volumes of all gases at the same temperature and pressure should contain an equal number of molecules".
• Law means that as long as the temperature and pressure remain constant, the volume depends upon the number of molecules of the gas or in other words amount of the gas.
• E.g: If 1 liter of H2 contains x molecules, then 1 liter of O2/Cl2/ any other gas will contain equal molecules, under similar conditions of temperature and pressure.
• Avogadro made a distinction between atoms and molecules.
• Since the volume of a gas is directly proportional to the number of moles; one mole of each gas at normal temperature and pressure (NTP) will have the same volume.
• i.e., one mole= Avagodro number of molecules (6.022 ×1023 molecules)
• Hence, at NTP, 22.4 L of H2/O2/Cl2/ any other gas contains 1 mole of the substance or NA molecules (Avagadro no: of molecules). 
• Mathematically, Avagadro's law is given by:

\(W \propto V\)       

⇒ \({\text{W = K V}}\)

⇒ \(\frac{{{W_1}}}{{{V_1}}} = \frac{{{W_2}}}{{{V_2}}}\)

Vapour Pressure:

• It is the ratio of the mass of one molecule of a substance vapour to the mass of hydrogen.
• Vapour density is related to the Molecular mass of a substance by the equation:
  • Molecular mass = 2 × Vapour density

Calculation:

Given:

• Mass of the substance = 10g
• Temperature = 298K
• Pressure = 1 atm
• Volume = 5.6 L

1-mole gas occupies 22.4 L of volume at NTP.

\(\frac{{{W_1}}}{{{V_1}}} = \frac{{{W_2}}}{{{V_2}}}\)

\(\frac{{{10}}}{{{5.6}}} = \frac{{{W_2}}}{{{22.4}}}\)

W2 = 40 

Molecular mass = 40g

The Correct Answer is Atom.

Since at the time of dalton the smallest particle is an atom So, the correct answer is an atom.

Key Points

John Dalton Postulates about atoms.

• All matter is made up of tiny, indivisible particles called atoms.
• All atoms of a specific element are identical in mass, size, and other properties. However, atoms of different elements exhibit different properties and vary in mass and size.
• Atoms can neither be created nor destroyed. Furthermore, atoms cannot be divided into smaller particles.
• Atoms of different elements can combine with each other in fixed whole-number ratios in order to form compounds.
• Atoms can be rearranged, combined, or separated in chemical reactions

Not all atoms exist free as in nature like O₂ N₂ but elements like Noble elements and gases like He, Argon etc can exist freely.

Additional Information

• A Molecule is an electrically neutral group of two or more atoms held together by chemical bonds. Molecules are distinguished from ions by their lack of electrical charge.
• Positively charged ions are called Cations. A cation has more protons than electrons, consequently giving it a net positive charge. 
• Negatively charged ions are called Anions. An anion has more electrons than protons, consequently giving it a net negative charge.

The correct answer is 12.

Explanation:

Number of grams of Helium (He) = 48 gm

Molecular mass is the sum of total protons and neutrons.

The molecular mass of Helium = 2 × 2 = 4

Now multiply the mass by the number of elements we get

⇒  4 × 1 = 4

One mole is equal to the total mass of the molecule.

We know one mole is equal to 4 gm of a Glucose molecule.

Now the number of moles in 48 gm of Helium is

⇒ (n) = 48/4

⇒ n = 12 moles

The number of moles in 48 gm is 12 moles.

The correct answer is 50.2 × 1023.

Concept:

Mole:

• It is the SI unit of the amount of substance.
• One mole contains exactly 6.022 ×10²³ atoms/molecules. 
• The mass of one mole of a substance in grams is called its molar mass.
• The molar mass in grams is numerically equal to atomic/molecular/formula mass in u.
• So, 1 mole = 6.022 × 1023 atoms/molecules = Atomic/molecular mass of an element/compound.

Explanation:

The molar mass of glucose (C₆H₁₂O₆) = 6 × 12 + 12 × 1 + 6 × 16 = 72 + 12 + 96 = 180g 

1 mole of glucose = molar mass of glucose = 6.022 × 10²³ molecules of glucose 

180g of glucose = 6.022 × 1023 molecules of glucose

1g of glucose = 6.022 × 1023/ 180 molecules of glucose 

250g of glucose = (250 × 6.022 × 1023)/ 180 molecules of glucose 

One molecule of glucose has six atoms of oxygen. 

So,  (250 × 6.022 × 1023) / 180 molecules of glucose will have atoms of oxygen

=  (6 × 250 × 6.022 × 1023) / 180 = 50.1833 × 1023 = 50.2 × 1023

250g of glucose will have 50.2 × 1023 atoms of oxygen.

Concept:

Mole Concept -

•  The quantity one mole of a substance signifies 6.022 × 1023 number of particles of that substance which may be atoms, molecules, or ions.
• The quantity is a universal constant like Dozen, Gross, etc., and is known as Avogadro number, denoted by NA . after the scientist Amedeo Avogadro.
• Examples- In one mole of H2, there are 6.022 × 1023 molecules of hydrogen, and the number of atoms is 2 × 6.022 × 1023, as one molecule of hydrogen contains two-atom each.
• The mass of one mole of a substance is called its Molar Mass (M) or Atomic mass expressed in grams.
• The volume occupied by a mole of gas is 22.4 L at NTP, called its Molar Volume.
• The no. of moles (n) is calculated as =

The number of particles / Avogadro’s number.

To summarise, we can say, 

Explanation:

Mass in an isolated system can neither be created nor destroyed.

During a chemical process, the mass of the elements is the same as before after the reaction.

So a chemical reaction is always balanced, i.e, the number of atoms before and after the reaction should be the same.

The balanced chemical equation for the reaction is:

PbS + 4O₃ → PbSO₄ + 4O₂

• In the above-balanced chemical reaction, we see that for oxidizing one mole of PbS, 4 moles of lead sulphide are required.
• In the reaction, 4 moles of oxygen is produced. Hence, the value of 'x' is 4, and the value of 'y' is also 4.
• The mass of 4 moles of oxygen = 4 × 32 = 128
• The mass of 4 moles of Ozone = 4 × 48 = 192.
• The value of 'x' = 192
• The value of 'y' = 128 

Hence the ratio of x:y = 192:128 = 3:2

The correct answer is 3.6 × 1024.Key Points

• CH₃OH is the chemical formula for methanol, which is a colorless liquid with a mild odor.
• One mole of any substance contains 6.02 x 10²³ particles, which is known as Avogadro's number.
• In CH₃OH, there are 6 atoms in one molecule: 1 carbon, 4 hydrogen, and 1 oxygen.
• To calculate the number of atoms in one mole of CH₃OH, we multiply Avogadro's number by the number of atoms in one molecule, which is 6.
• Hence, the correct answer is 6.02 x 10²³ x 6 = 3.6 x 10²⁴ atoms in one mole of CH₃OH.​

Additional Information

• ​Usually, the Avogadro constant is represented by NA or L.
• The quantity of substance in a sample, which is calculated by dividing the number of constituent particles by NA, uses it as a normalizing factor.
• Depending on the substance and the type of reaction, the units could be molecules, atoms, ions, or electrons.

Correct answer: 3)

Concept:

• Compound: When two or more atoms of different elements combine together in a definite ratio, the molecule of a compound is obtained.
• Molecule: A molecule consisting of atoms of only one element is therefore not a compound.
• A chemical compound is composed of many identical molecules composed of atoms from more than one element held together by chemical bonds.
• A mixture and a compound are two different entities because a mixture is formed when two or more substances are physically mixed together.

Explanation:

• As we know a compound is a material formed by chemically bonding two or more chemical elements.
• The constituents of a compound cannot be separated into simpler substances by physical methods. 
• They can be separated by chemical methods. 
• The atoms of different elements are present in a compound in a fixed and definite ratio and this ratio is characteristic of a particular compound.
• The properties of a compound are different from those of its constituent elements.
• A compound has different physical properties from its constituent elements.
• For example,  a water molecule(solid, liquid, or gas) comprises two hydrogen atoms(gases) and one oxygen atom(gases). Similarly, a molecule of carbon dioxide contains two oxygen atoms combined with one carbon atom. carbon dioxide has different properties than carbon and oxygen.

Conclusion:

• Thus, we can conclude that a compound has different physical properties from its constituent elements.
• A compound does not retain the physical properties of its constituent elements.
• Because when two or more elements get combined chemically, they form a new compound with new chemical and physical properties depending on the nature of bonding in the compound.

Additional Information
Element:  I used one type of coloured m and m to represent that elements are made up of one type of atom

Molecule: I positioned together to show that molecules are made of atoms chemically bonded together

Compound: I used two different types of m and m positioned together to show compounds are two or more types of atom joined with a chemical bond

Mixture: I used several types of m and m. They are positioned separately to show mixtures contain elements and compounds not joined with a chemical bond

The correct answer is 111 u.

Concept:

The sum of the atomic masses of all the atoms in a molecule of the substance is Molecular mass.

• It is calculated in practice by summing the atomic weights of the atoms making up the substance’s molecular formula
• For example, the molecular mass of water (H2O), which has two atoms of hydrogen and one atom of oxygen, is

= 2 x mass of hydrogen atom + mass of oxygen atom

= 2 x 1 + 16

= 2 + 16

= 18

• The formula mass of a molecule is the sum of the atomic weights of the atoms in the empirical formula of the compound.
• Mole corresponds to the mass of a substance that contains 6.023 x 1023 particles of the substance.
• Atomic mass is the mass of a single atom of a chemical element.

Explanation:

• The formula unit mass of CaCl2 is 111 u.
• Atomic Mass of Ca= 40
• Atomic Mass of Chlorine= 35.5 x2 = 71
• Formula Unit Mass = 40+71= 111.
• Formula unit mass is defined as the sum of the mass of all the atoms each multiplied by atomic masses that are present in the empirical formula of a compound.
• Cacl2 or calcium chloride is also known as Ice Bite.

Concept:

1. Grams = moles × atomic weight
2. Atomic weight = number of neutrons + number of protons
3. The atomic weight of the sodium atom is 23.

Calculation:

Given that,

There is a 1 gram-atom of sodium. Therefore,

By using the above concept

Grams = 1 mole × atomic weight 

Since atomic weight = 23 

Therefore,

Grams = 1 mole ×  23 = 23 grams

∴ 23 grams are contained in 1 gram-atom of a sodium atom.