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Q1. 1 g of hydrogen contains
1 g of hydrogen contains 1 mole of atoms. The atomic mass of hydrogen is 1 u, so its molar mass is 1 g/mol. This means that 1 gram of hydrogen atoms is exactly equal to 1 mole of hydrogen atoms (6.022 × 10²³ atoms). This is a direct application of the mole concept, where the mass in grams equals the atomic mass in u.
Q2. The formula unit mass of NaCl is
The formula unit mass of NaCl is 58.5 u. Sodium (Na) has an atomic mass of 23 u and chlorine (Cl) has an atomic mass of 35.5 u. Adding them together: 23 + 35.5 = 58.5 u. This is the mass of one formula unit of sodium chloride.
Q3. A chemical formula represents the
A chemical formula represents the composition of a compound. It tells us the types of elements present and the ratio of their atoms in the compound. For example, H₂O tells us that water contains hydrogen and oxygen in a 2:1 ratio. The formula does not indicate colour, state, or density.
Q4. The atomic mass of sodium is
The atomic mass of sodium is 23 u (approximately 22.99 u). Sodium has an atomic number of 11 and is a highly reactive metal. Its atomic mass of 23 makes it a light metal, and this value is essential for calculating formula masses of sodium compounds.
Q5. CaCl₂ contains how many chlorine atoms?
CaCl₂ contains two chlorine atoms. The formula shows that there is one calcium atom (Ca) and two chlorine atoms (Cl₂) in each formula unit of calcium chloride. This is because calcium has a valency of +2 and chlorine has a valency of -1, requiring two chlorine atoms.
Q6. The molecular mass of water in grams is
The molecular mass of water (H₂O) is 18 u, which corresponds to 18 grams per mole (18 g/mol). The atomic masses are: H = 1 u (2 atoms = 2 u) and O = 16 u. Total = 2 + 16 = 18 u. This means that 1 mole of water weighs 18 grams.
Q7. The atomic mass of nitrogen is taken as
The atomic mass of nitrogen is 14 u. Nitrogen has an atomic number of 7 and is a non-metal that forms about 78% of the Earth’s atmosphere. Its atomic mass is used in many calculations, including the molecular mass of ammonia (NH₃), which is 17 u.
Q8. The quantity of a substance can be expressed in terms of
The quantity of a substance can be expressed in terms of mass or the number of particles. Mass is a macroscopic measure, while the number of particles (atoms, molecules, ions) is a microscopic measure. The mole concept connects these two ways of expressing quantity.
Q9. In the reaction, two molecules of hydrogen combine with
In the reaction 2H₂ + O₂ → 2H₂O, two molecules of hydrogen combine with one molecule of oxygen. This shows that hydrogen and oxygen react in a 2:1 ratio to form water. This is a balanced chemical equation that follows the law of constant proportions.
Q10. One mole of hydrogen atoms contains
One mole of hydrogen atoms contains 6.022 × 10²³ atoms. This is Avogadro’s number. Whether it is atoms, molecules, or ions, one mole of any substance contains this fixed number of entities. This makes the mole a very useful concept in chemistry.
Q11. Formula unit mass is used for substances made of
Formula unit mass is used for substances made of ions (ionic compounds). In ionic compounds like NaCl or CaCl₂, the particles are ions, not molecules. The term “formula unit” is used instead of “molecule” because ionic compounds do not exist as discrete molecules.
Q12. The symbol for mole is
The symbol for mole is “mol”. This is the SI unit for the amount of substance. It is a standard unit used in chemistry to count particles. One mole contains Avogadro’s number of entities (6.022 × 10²³). The symbol is always written as “mol”.
Q13. Molecular mass represents the relative mass of a
Molecular mass represents the relative mass of a molecule. It is the sum of the atomic masses of all the atoms in the molecule. For example, the molecular mass of water (H₂O) is the sum of two hydrogen atoms (2 u) and one oxygen atom (16 u) = 18 u.
Q14. 16 u of oxygen corresponds to
16 u of oxygen corresponds to one atom of oxygen because the atomic mass of oxygen is 16 u. This is the mass of a single oxygen atom. One molecule of O₂ would have a mass of 32 u. One mole of oxygen atoms would be 16 grams, not 16 u.
Q15. One mole of any substance contains the same number of
One mole of any substance contains the same number of atoms or molecules (or ions), which is Avogadro’s number (6.022 × 10²³). This is true regardless of the substance. The number of grams, electrons, or protons in a mole varies depending on the substance.
Q16. The number 6.022 × 10²³ is called the
The number 6.022 × 10²³ is called the Avogadro number or Avogadro constant. It is named after the Italian scientist Amedeo Avogadro. This number represents the number of particles (atoms, molecules, ions) in one mole of any substance.
Q17. The total mass of water formed in the reaction is
In the reaction 2H₂ + O₂ → 2H₂O, two molecules of water are formed. Each water molecule (H₂O) has a mass of 18 u (2×1 + 16). So two water molecules have a total mass of 2 × 18 = 36 u. This follows the Law of Conservation of Mass.
Q18. The molecular mass of nitric acid (HNO₃) is
The molecular mass of nitric acid (HNO₃) is 63 u. Hydrogen (H) = 1 u, nitrogen (N) = 14 u, and oxygen (O) = 16 u (×3 = 48 u). Total = 1 + 14 + 48 = 63 u. Nitric acid is a strong acid used in many industrial processes.
Q19. The atomic mass of chlorine is
The atomic mass of chlorine is 35.5 u. This is an average value due to the presence of two isotopes: chlorine-35 (about 75%) and chlorine-37 (about 25%). The average atomic mass is 35.5 u. This is why many calculations involving chlorine use 35.5 rather than a whole number.
Q20. One mole contains exactly
One mole contains exactly 6.022 × 10²³ entities (atoms, molecules, ions, etc.). This number is known as Avogadro’s number. It is a fundamental constant in chemistry and is used to convert between the number of particles and the amount of substance in moles.
Q21. The molecular mass of water (H₂O) is
The molecular mass of water (H₂O) is 18 u. Hydrogen has an atomic mass of 1 u (2 atoms = 2 u) and oxygen has an atomic mass of 16 u. Adding them together: 2 + 16 = 18 u. This is the mass of one molecule of water.
Q22. The atomic mass of oxygen used in calculations is
The atomic mass of oxygen used in calculations is 16 u. Oxygen has an atomic number of 8 and is a non-metal that is essential for life. Its atomic mass is used in many calculations, from water to carbon dioxide to organic compounds.
Q23. Formula unit mass is calculated in the same way as
Formula unit mass is calculated in the same way as molecular mass. Both involve adding the atomic masses of all atoms in the formula. The difference is that formula unit mass is used for ionic compounds (where no molecules exist), while molecular mass is used for covalent compounds.
Q24. The molar mass of a molecule is also called
The molar mass of a molecule is also called gram molecular mass. It is the mass of one mole of the substance expressed in grams. For example, the molecular mass of water is 18 u, and its molar mass is 18 g/mol. This is the same numerical value but in grams per mole.
Q25. Molecular mass is defined as the sum of the
Molecular mass is defined as the sum of the atomic masses of all the atoms in a molecule. For example, to find the molecular mass of H₂O, we add the atomic masses of two hydrogen atoms (2 × 1 u) and one oxygen atom (16 u), giving 18 u.
Q26. The Avogadro constant is represented by
The Avogadro constant is represented by Nₐ. The subscript “A” stands for Avogadro. This constant has a value of 6.022 × 10²³ mol⁻¹. It is one of the fundamental constants in chemistry and is used to convert between the number of particles and the amount of substance.
Q27. One dozen represents
One dozen represents 12 items. This is a commonly used counting unit in everyday life, similar to how a “mole” is used in chemistry. One dozen eggs means 12 eggs, just as one mole of atoms means 6.022 × 10²³ atoms.
Q28. The charge on the phosphate (PO₄³⁻) ion is
The charge on the phosphate ion (PO₄³⁻) is -3. This polyatomic ion consists of one phosphorus atom and four oxygen atoms with an overall charge of -3. It is common in biological systems (like ATP) and in fertilizers.
Q29. Molecular mass is expressed in the unit
Molecular mass is expressed in the unit u (unified atomic mass unit). This unit is used because molecular masses are relative masses compared to 1/12th of the mass of a carbon-12 atom. The molar mass (in grams) has the same numerical value but is expressed in g/mol.
Q30. The PO₄³⁻ ion contains how many oxygen atoms?
The PO₄³⁻ ion contains four oxygen atoms. The subscript 4 after O indicates the number of oxygen atoms in the phosphate ion. It also contains one phosphorus atom. This ion is an example of a polyatomic ion with a -3 charge.
Q31. The SI unit of amount of substance is
The SI unit of amount of substance is the mole (symbol: mol). It is one of the seven base SI units. The mole is defined as the amount of substance that contains the same number of entities as there are atoms in 0.012 kg of carbon-12.
Q32. The reaction 2H₂ + O₂ → 2H₂O shows the law of
The reaction 2H₂ + O₂ → 2H₂O shows the law of conservation of mass. The total mass of reactants (2H₂ + O₂) equals the total mass of products (2H₂O). This reaction also demonstrates the law of constant proportions (water always has the same ratio of H to O).
Q33. 16 g of oxygen contains
16 g of oxygen contains one mole of oxygen atoms. Since the atomic mass of oxygen is 16 u, its molar mass is 16 g/mol. This means 16 grams of oxygen atoms is exactly 1 mole. However, one mole of oxygen molecules (O₂) would weigh 32 g.
Q34. The mass of one mole of a substance is equal to its
The mass of one mole of a substance is equal to its relative atomic or molecular mass expressed in grams. For example, the relative atomic mass of hydrogen is 1 u, so one mole of hydrogen atoms weighs 1 gram. Similarly, the molecular mass of water is 18 u, so one mole of water weighs 18 grams.
Q35. How many hydrogen atoms are present in an H₂S molecule?
An H₂S molecule contains two hydrogen atoms. The subscript 2 after H indicates the number of hydrogen atoms in the molecule. This compound also contains one sulphur atom. H₂S is hydrogen sulphide, a gas with a rotten egg smell.
Q36. Sodium chloride exists as
Sodium chloride exists as ions (Na⁺ and Cl⁻). It is an ionic compound formed by the transfer of electrons from sodium to chlorine. In solid form, it exists as a crystal lattice of these ions, not as discrete molecules. This is why we use “formula unit” instead of “molecule.”
Q37. The atomic mass of hydrogen used in calculations is
The atomic mass of hydrogen used in calculations is 1 u (approximately 1.008 u). Hydrogen is the lightest element, and its atomic mass serves as the basis for comparing other elements. It is a fundamental constant in chemistry.
Q38. The gram atomic mass of hydrogen is
The gram atomic mass of hydrogen is 1 g. This is because the atomic mass of hydrogen is 1 u, and the molar mass is 1 g/mol. Therefore, one mole of hydrogen atoms weighs 1 gram. Hydrogen gas (H₂) would have a gram molecular mass of 2 g.
Q39. How many sulphur atoms are present in an H₂S molecule?
An H₂S molecule contains one sulphur atom. The formula shows S (without any subscript), indicating that there is only one sulphur atom in the molecule. This is a simple molecule containing two hydrogen atoms and one sulphur atom.
Q40. Molar mass is expressed in the unit
Molar mass is expressed in the unit g mol⁻¹ (grams per mole). This unit indicates the mass of one mole of a substance in grams. For example, the molar mass of water is 18 g mol⁻¹, meaning that one mole of water weighs 18 grams. The molecular mass is expressed in u.
Q41. The term chemical formula refers to the symbolic representation of a compound’s
The term chemical formula refers to the symbolic representation of a compound’s composition. It tells us the types of elements present and the ratio of their atoms. For example, the formula H₂O shows that water is composed of hydrogen and oxygen in a 2:1 ratio.
Q42. Formula unit mass is applicable to substances that exist as
Formula unit mass is applicable to substances that exist as ions (ionic compounds). Ionic compounds like NaCl, CaCl₂, and MgO do not exist as discrete molecules. Instead, they exist as a lattice of ions, so the term “formula unit” is used to describe their composition.
Q43. Molecular mass is applicable to substances that exist as
Molecular mass is applicable to substances that exist as molecules (covalent compounds). Compounds like water (H₂O), carbon dioxide (CO₂), and glucose (C₆H₁₂O₆) are molecular substances, so their masses are called molecular masses.
Q44. 4 u of hydrogen combine with how many units of oxygen?
4 u of hydrogen combine with 32 u of oxygen to form water. In water (H₂O), the mass ratio of hydrogen to oxygen is 2:16, which simplifies to 1:8. So 4 u of hydrogen requires 32 u of oxygen (4 × 8 = 32). This follows the Law of Constant Proportions.
Q45. Changing the unit from u to g while keeping the number same gives
Changing the unit from u to g while keeping the number same gives the molar mass. For example, if the molecular mass of water is 18 u, then the molar mass is 18 g/mol. The numerical value remains the same; only the unit changes from “u” to “g/mol.”
Q46. The concept of mole makes it easier to count particles by
The concept of mole makes it easier to count particles by using mass. Instead of counting individual atoms or molecules, which is impossible, we can measure the mass of a substance and relate it to the number of particles using Avogadro’s number. One mole equals the atomic/molecular mass in grams.
Q47. One gross represents
One gross represents 144 items. This is a traditional counting unit used for items like pencils or eggs. Just as “dozen” means 12, “gross” means 144. This is similar to how the mole is used in chemistry to count particles.
Q48. The formula unit mass of CaCl₂ is
The formula unit mass of CaCl₂ is 111 u. Calcium (Ca) has an atomic mass of 40 u and chlorine (Cl) has an atomic mass of 35.5 u. In CaCl₂, there is one Ca atom (40 u) and two Cl atoms (2 × 35.5 = 71 u). Total = 40 + 71 = 111 u.
Q49. The symbol used for amount of substance is
The symbol used for amount of substance is n. It is the standard notation for the number of moles in chemistry. For example, n(H₂O) = 2 mol means there are 2 moles of water. This is used in various calculations involving moles, mass, and Avogadro’s number.
Q50. A chemical equation directly indicates the number of
A chemical equation directly indicates the number of atoms or molecules involved in a reaction. For example, 2H₂ + O₂ → 2H₂O shows that 2 molecules of hydrogen react with 1 molecule of oxygen to form 2 molecules of water. The coefficients represent the relative number of molecules.