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

CONCEPT:

Equal Number of Atoms

• The number of atoms in a given sample depends on the number of moles of molecules present, as well as the number of atoms in each molecule.
• The number of moles can be calculated using the formula:

  Moles = Mass / Molar Mass

• Once the moles are determined, the number of molecules can be calculated using Avogadro's number:

  Number of molecules = Moles × 6.022 × 10²³

• To calculate the total number of atoms, multiply the number of molecules by the number of atoms in the molecular formula.

EXPLANATION:

• For each option, calculate the number of moles and the total number of atoms:
  • A. 212 g of Na₂CO₃:
    • Moles = 212 / 106 = 2 moles
    • Na₂CO₃ contains 6 atoms per molecule (2 Na, 1 C, 3 O).
    • Total atoms = 2 × 6 × 6.022 × 10²³ = 72.264 × 10²³ atoms
  • B. 248 g of Na₂O:
    • Moles = 248 / 62 = 4 moles
    • Na₂O contains 3 atoms per molecule (2 Na, 1 O).
    • Total atoms = 4 × 3 × 6.022 × 10²³ = 72.264 × 10²³ atoms
  • C. 240 g of NaOH:
    • Moles = 240 / 40 = 6 moles
    • NaOH contains 3 atoms per molecule (1 Na, 1 O, 1 H).
    • Total atoms = 6 × 3 × 6.022 × 10²³ = 108.396 × 10²³ atoms
  • D. 12 g of H₂:
    • Moles = 12 / 2 = 6 moles
    • H₂ contains 2 atoms per molecule (2 H).
    • Total atoms = 6 × 2 × 6.022 × 10²³ = 72.264 × 10²³ atoms
  • E. 220 g of CO₂:
    • Moles = 220 / 44 = 5 moles
    • CO₂ contains 3 atoms per molecule (1 C, 2 O).
    • Total atoms = 5 × 3 × 6.022 × 10²³ = 90.33 × 10²³ atoms
• Comparing the total atoms A, B, and D have equal total atoms (72.264 × 10²³).

Therefore, the correct answer is Option 2: A, B, and D only.

CONCEPT:

Dalton's Atomic Theory and Its Limitations

• Dalton's Atomic Theory proposed that:
  • All matter is made up of tiny, indivisible particles called atoms.
  • Atoms of the same element are identical in mass and properties.
  • Atoms of different elements combine in fixed ratios to form compounds (Law of Constant Proportion).
  • Atoms are neither created nor destroyed in chemical reactions (Law of Conservation of Mass).
• Limitations:
  • While Dalton's theory could explain many laws like the Law of Conservation of Mass and the Law of Constant Proportion, it failed to explain the Law of Gaseous Volume.
  • The Law of Gaseous Volume, proposed by Gay-Lussac, states that gases react in simple whole-number ratios by volume under the same conditions of temperature and pressure. This requires understanding molecular structures, which Dalton's theory did not address.

EXPLANATION:

• The Law of Gaseous Volume involves the concept of molecules and their behavior in the gaseous state. For example:
  • When hydrogen gas reacts with oxygen gas to form water vapor:

    2H₂(g) + O₂(g) → 2H₂O(g)

  • The volume ratio of hydrogen, oxygen, and water vapor is 2:1:2 under the same conditions.
• Dalton's Atomic Theory assumed atoms as indivisible and did not account for the existence of molecules or the behavior of gases in terms of volume. Thus, it could not explain Gay-Lussac's Law of Gaseous Volume.

Therefore, Dalton's Atomic Theory could not explain the Law of Gaseous Volume.

CONCEPT:

Normality (N) of a solution

• Normality (N) is a measure of concentration equivalent to molarity (M) multiplied by the number of equivalents per mole of solute.
• For oxalic acid (H₂C₂O₄), which is a diprotic acid (provides 2 H⁺ ions), the number of equivalents per mole is 2.

EXPLANATION:

• Given:
  • Molecular weight of oxalic acid (H₂C₂O₄) = 126 g/mol
  • Volume of solution = 100 ml = 0.1 L
  • Desired normality (N) = 0.1 N
• To find the mass of oxalic acid required to prepare the solution, use the formula:
  • Mass (g) = Normality (N) × Equivalent weight (g/equiv) × Volume (L)
• First, calculate the equivalent weight:
  • Equivalent weight = Molecular weight / Number of equivalents per mole
  • Equivalent weight = 126 g/mol / 2 = 63 g/equiv
• Now, calculate the mass:
  • Mass (g) = 0.1 N × 63 g/equiv × 0.1 L
  • Mass (g) = 0.63 g

Therefore, 0.63 g of solid oxalic acid is required to prepare 100 ml of exactly 0.1 N oxalic acid solution in water.

CONCEPT:

Nomenclature of Alkenes and Alkynes

• The IUPAC nomenclature system is a standardized method for naming chemical compounds.
• When naming compounds with both double (alkene) and triple (alkyne) bonds, certain rules must be followed:
  • Identify the longest continuous carbon chain that contains both the double and triple bonds.
  • Number the chain from the end nearest to the first multiple bond (double or triple).
  • Indicate the positions of the double and triple bonds using the appropriate locants (numbers).
  • Write the name as a single word with the position and type of the bonds in the format: [prefix] - [locant]-[alken]-[locant]-[yne].

EXPLANATION:

• For the compound CH₃-CH=CH-C≡CH:
  • The longest carbon chain containing both the double and triple bonds has 5 carbons.
  • Numbering from the end closest to the first multiple bond gives the locants as follows:
    • Double bond (C=C) between C-2 and C-3.
    • Triple bond (C≡C) between C-4 and C-5.
  • Combining the locants with the root name "pent" (for 5 carbons) results in the name: Pent-3-en-1-yne.

Therefore, the correct IUPAC name of the compound CH₃-CH=CH-C≡CH is Pent-3-en-1-yne.

CONCEPT:

Unitless Terms in Chemistry

• In chemistry, certain quantities are considered unitless because they represent ratios or pure numbers without any associated units.

EXPLANATION:

• Among the given options:
  • Molality (m) is defined as the number of moles of solute per kilogram of solvent. It has units of moles per kilogram (mol/kg).
  • Molarity (M) is defined as the number of moles of solute per liter of solution. It has units of moles per liter (mol/L).
  • Mole fraction (χ) is defined as the ratio of the number of moles of a component to the total number of moles of all components in the mixture. It is a ratio and therefore unitless.
  • Strength typically refers to concentration and can have various units such as grams per liter (g/L) or moles per liter (mol/L).
• Therefore, the only term that is unitless is the mole fraction (χ).

Therefore, the correct answer is Mole fraction.

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.

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.

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

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.