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

CONCEPT:

Neutralization of a Monobasic Acid with a Base

• In the neutralization reaction between a strong acid and a base, the number of moles of acid is equal to the number of moles of base at the equivalence point.
• The formula to calculate the volume required for neutralization is:

  Volume of acid (L) = (moles of base) / (molarity of acid)

• The number of moles of KOH is calculated as:

  moles of KOH = Molarity × Volume

• For the acid, the molarity is calculated using its molar mass and density.

EXPLANATION:

• Given:
  • Molar mass of the acid = 36.5 g/mol
  • Density of the acid = 1.03 kg/L
  • Volume of KOH solution = 50 mL = 0.050 L
  • Molarity of KOH = 0.45 M
• Step 1: Calculate the moles of KOH:

  moles of KOH = Molarity × Volume = 0.45 × 0.050 = 0.0225 mol

• Step 2: Calculate the molarity of the acid: -
  • Density of the acid solution = 1.03 kg/L = 1030 g/L -
  • Molar mass of the acid = 36.5 g/mol -
  • Moles of acid per liter = 1030 g / 36.5 g/mol = 28.2 mol/L
  • So, the molarity of the acid is 28.2 M.
• Step 3: Use the neutralization equation: -
  • The moles of acid required = moles of KOH (since it’s monobasic) -
  • Volume of the acid required = moles of KOH / Molarity of the acid
  • = 0.0225 mol / 28.2 M = 0.000797 L = 0.797 mL
• Step 4: Express the final result in × 10⁻² mL:

  0.797 mL = 7.97 × 10⁻² mL (Nearest integer: 8 × 10⁻² mL)

The volume of the acid required is 8 × 10⁻² mL.

CONCEPT:

Calculating the Molar Mass and Weight of 0.1 mol of a Compound

• The molar mass of a compound is the sum of the atomic masses of all the elements in the compound, multiplied by the number of atoms of each element.
• The weight of 0.1 mol of the compound is then calculated by multiplying the molar mass by 0.1.

EXPLANATION:

• The compound contains:
  • 13 Carbon (C) atoms
  • 20 Hydrogen (H) atoms
  • 6 Nitrogen (N) atoms
  • 4 Oxygen (O) atoms
• Calculate the molar mass of the compound:

  Molar mass = (13 × 12) + (20 × 1) + (6 × 14) + (4 × 16) = 324 g/mol

• Now, calculate the weight of 0.1 mol:

  Weight = 0.1 × 324 = 32.4 g

Therefore, 0.1 mol of the given compound weighs 32.4 grams.

The Correct answer is C₄H₈.

Key Points

• The question asks us to determine the molecular formula of a compound with an empirical formula CH₂ and a molar mass of 56 g/mol.
• The empirical formula represents the simplest ratio of atoms in a compound. In this case, CH₂ indicates there is 1 carbon atom for every 2 hydrogen atoms.
• The molar mass of CH₂ is calculated as follows:
  • Carbon (C): Atomic mass = 12 g/mol.
  • Hydrogen (H): Atomic mass = 1 g/mol (for 2 hydrogen atoms, total = 2 g/mol).
  • Total molar mass of CH₂ = 12 + 2 = 14 g/mol.
• To find the molecular formula, divide the molar mass of the compound (56 g/mol) by the molar mass of the empirical formula (14 g/mol):
  • 56 ÷ 14 = 4.
• This means the molecular formula is 4 times the empirical formula.
• Multiply CH₂ by 4 to get the molecular formula:
  • Carbon atoms: 1 × 4 = 4.
  • Hydrogen atoms: 2 × 4 = 8.
  • Molecular formula = C₄H₈.
• Thus, the correct molecular formula of the compound is C₄H₈.

 Additional Information

• Empirical Formula
  • The empirical formula represents the simplest whole-number ratio of atoms in a compound.
  • For example, CH₂ means that for every 1 carbon atom, there are 2 hydrogen atoms.
  • It does not provide the exact number of atoms in the molecule, which is determined by the molecular formula.
• Molecular Formula
  • The molecular formula gives the actual number of atoms of each element in a molecule.
  • It is a multiple of the empirical formula, determined by dividing the compound’s molar mass by the empirical formula mass.
  • For example, if the empirical formula is CH₂ and the molar mass is 56 g/mol, the molecular formula is C₄H₈.
• Incorrect Options
  • CH₂: This is the empirical formula, not the molecular formula.
  • C₃H₆: While this follows a similar ratio (1:2), its molar mass is 42 g/mol, which does not match the given molar mass of 56 g/mol.
  • C₂H₄: This also follows the 1:2 ratio, but its molar mass is only 28 g/mol, which is incorrect.

The correct answer is 1.2044 × 1024.

Key Points

• The number of molecules in a substance is calculated using Avogadro's number (6.022 × 10²³ molecules/mol).
• In this question, we are given 2 moles of CO₂.
• To find the number of molecules, multiply the number of moles by Avogadro's number: 2 × 6.022 × 10²³ = 12.044 × 10²³ = 1.2044 × 10²⁴ molecules.
• Hence, 12.044 × 10²³ molecules are present in 2 moles of CO₂.
• This calculation is based on the fundamental concept of mole and Avogadro's constant.

Additional Information

• Avogadro's Number:
  • Avogadro's number (6.022 × 10²³) represents the number of particles (atoms, molecules, or ions) in one mole of a substance.
  • It is a fundamental constant in chemistry.
• Mole Concept:
  • A mole is a unit in chemistry used to express amounts of a chemical substance.
  • One mole of any substance contains exactly 6.022 × 10²³ particles.
• Carbon Dioxide (CO₂):
  • It is a molecule composed of one carbon atom and two oxygen atoms.
  • It is a major greenhouse gas and is essential for the process of photosynthesis in plants.
• Applications of Avogadro's Number:
  • Used to calculate the number of particles in a given amount of substance.
  • Helps in determining molar mass and molecular formulas in stoichiometry.

EXPLANATION:

• A. 50 g of H₂ (g):

  Number of moles = 50 g / 2 g/mol = 25 moles

  Number of molecules = 25 moles × 6.022 × 10²³ molecules/mol
  Number of atom= 2 x 25 moles × 6.022 × 1023 molecules/mol

• B. 44 g of CO₂ (g):

  Number of moles = 44 g / 44 g/mol = 1 mole

  Number of molecules = 1 mole × 6.022 × 10²³ molecules/mol
  Number of atom = 3 x 1 mole × 6.022 × 1023 molecules/mol

• C. 18 g of H₂O (l):

  Number of moles = 18 g / 18 g/mol = 1 mole

  Number of molecules = 1 mole × 6.022 × 10²³ molecules/mol
  Number of atom = 3 x 1 mole × 6.022 × 1023 molecules/mol

• D. 88 g of NH₃ (g):

  Number of moles = 88 g / 17 g/mol = 5.18 moles

  Number of molecules = 5.18 moles × 6.022 × 10²³ molecules/mol
  Number of atom = 4 x 5.18 moles × 6.022 × 10²³ molecules/mol

• E. 160 g of O₂ (g):

  Number of moles = 160 g / 32 g/mol = 5 moles

  Number of molecules = 5 moles × 6.022 × 10²³ molecules/mol
  Number of atoms = 2 x 5 moles × 6.022 × 10²³ molecules/mol

• Comparing the number of molecules for each, we see that:
  • B and C both have 1 mole of molecules.
  • Therefore, the correct answer is B and C only.

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.