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Q1. A compound having metal and non-metal is generally
A compound having a metal and a non-metal is generally ionic. In such compounds, the metal loses electrons to form positive ions (cations) and the non-metal gains electrons to form negative ions (anions). This transfer of electrons results in the formation of ionic bonds. Examples include NaCl (sodium chloride) and CaO (calcium oxide).
Q2. Al₂(SO₄)₃ is named as
Al₂(SO₄)₃ is named aluminium sulphate. It is a compound formed from aluminium ions (Al³⁺) and sulphate ions (SO₄²⁻). The formula indicates that two aluminium ions combine with three sulphate ions to balance the charges. This is a common chemical used in water purification.
Q3. Sodium sulphide is represented by
Sodium sulphide is represented by Na₂S. Sodium has a valency of +1 and sulphide ion has a valency of -2. To balance the charges, two sodium ions (+1 each) are needed for every sulphide ion (-2). This gives the formula Na₂S.
Q4. In compounds containing polyatomic ions, the number of ions is shown by
In compounds containing polyatomic ions, the number of ions is shown by brackets and subscripts. For example, in Ca(OH)₂, the brackets around OH indicate that the hydroxide ion is a single polyatomic ion, and the subscript 2 shows that there are two such ions. This is necessary because polyatomic ions contain multiple atoms.
Q5. Calcium hydroxide is represented as
Calcium hydroxide is represented as Ca(OH)₂. Calcium has a valency of +2 and hydroxide ion (OH⁻) has a valency of -1. Two hydroxide ions are needed to balance one calcium ion, so brackets are used to enclose the polyatomic hydroxide ion, followed by the subscript 2.
Q6. The crossover method is used to
The crossover method is used to write chemical formulae. In this method, the valency of the cation is written as the subscript for the anion and vice versa. This ensures that the charges balance to form a neutral compound. For example, to write the formula of magnesium chloride, the valency of Mg (2) becomes the subscript for Cl, and the valency of Cl (1) becomes the subscript for Mg, giving MgCl₂.
Q7. Hydrogen chloride has the chemical formula
Hydrogen chloride has the chemical formula HCl. Hydrogen has a valency of +1 and chlorine has a valency of -1. They combine in a 1:1 ratio to balance the charges, giving the formula HCl. This is a simple binary compound formed from a non-metal (hydrogen) and a halogen (chlorine).
Q8. Brackets are used in Ca(OH)₂ because
Brackets are used in Ca(OH)₂ because hydroxide (OH⁻) is a polyatomic ion. Polyatomic ions contain more than one atom, and brackets are used to show that the entire group acts as a single unit. The subscript 2 outside the bracket indicates that there are two hydroxide ions.
Q9. Magnesium hydroxide has the formula
Magnesium hydroxide has the formula Mg(OH)₂. Magnesium has a valency of +2 and hydroxide ion (OH⁻) has a valency of -1. Two hydroxide ions are needed to balance the +2 charge of magnesium. The brackets are used to enclose the polyatomic hydroxide ion.
Q10. In magnesium chloride, the cation is
In magnesium chloride, the cation is Mg²⁺. A cation is a positively charged ion. Magnesium loses two electrons to form Mg²⁺, making it the cation. Chlorine is the anion (Cl⁻). The formula of magnesium chloride is MgCl₂.
Q11. K₂SO₄ is the formula of
K₂SO₄ is the formula of potassium sulphate. Potassium ions (K⁺) combine with sulphate ions (SO₄²⁻). Two potassium ions are needed to balance one sulphate ion. This is a common salt used as a fertilizer and in various chemical applications.
Q12. Brackets are not required in a formula when the number of polyatomic ions is
Brackets are not required in a formula when the number of polyatomic ions is one. For example, NaOH (sodium hydroxide) does not require brackets because there is only one hydroxide ion. Brackets are used only when there are two or more polyatomic ions in the formula (like in Ca(OH)₂).
Q13. Sodium carbonate has the formula
Sodium carbonate has the formula Na₂CO₃. Sodium has a valency of +1 and carbonate ion (CO₃²⁻) has a valency of -2. Two sodium ions (+1 each) balance one carbonate ion (-2). This is a common compound used in washing soda and glass making.
Q14. In calcium oxide (CaO), oxygen is written on the right because it is a
In calcium oxide (CaO), oxygen is written on the right because it is a non-metal. In chemical formulas of compounds, the metal is written first, followed by the non-metal. Calcium is a metal and oxygen is a non-metal, so the formula is written as CaO.
Q15. Roman numerals indicating valency are written in
Roman numerals indicating valency are written in round brackets. For example, iron(III) chloride indicates that iron has a valency of 3. This system is used to show the valency of elements that show variable valency, such as iron, copper, and mercury.
Q16. The anion in magnesium chloride is
The anion in magnesium chloride is Cl⁻. An anion is a negatively charged ion. Chlorine gains one electron to form a chloride ion (Cl⁻). In magnesium chloride (MgCl₂), there are two chloride ions for each magnesium ion.
Q17. The subscript 2 in Ca(OH)₂ indicates
The subscript 2 in Ca(OH)₂ indicates two hydroxide groups. The brackets enclose the entire OH group, and the subscript 2 applies to the whole group. This means there are two hydroxide ions (OH⁻) in calcium hydroxide.
Q18. NaOH does not require brackets because
NaOH does not require brackets because it has only one polyatomic ion (OH⁻). Since there is only one hydroxide ion, there is no need for brackets. The formula is simply written as NaOH. Brackets are used only when there is more than one polyatomic ion.
Q19. Mg(OH)₂ contains how many hydroxide ions?
Mg(OH)₂ contains two hydroxide ions. The subscript 2 outside the bracket indicates that there are two OH⁻ ions in the compound. This is because magnesium has a +2 valency, requiring two hydroxide ions (each -1) to balance the charge.
Q20. The valency of aluminium is
The valency of aluminium is 3. Aluminium has three electrons in its valence shell and loses all three to achieve a stable configuration. This gives it a +3 valency. This is why aluminium forms compounds like AlCl₃ and Al₂O₃.
Q21. Ammonium sulphate contains how many ammonium ions?
Ammonium sulphate contains two ammonium ions. Its formula is (NH₄)₂SO₄. The subscript 2 outside the bracket indicates that there are two ammonium ions (NH₄⁺) for every one sulphate ion (SO₄²⁻) to balance the charges.
Q22. Compounds made of two different elements are called
Compounds made of two different elements are called binary compounds. These compounds consist of atoms of two different elements chemically combined. Examples include NaCl (sodium chloride), H₂O (water), and CaO (calcium oxide). Binary compounds can be ionic or covalent.
Q23. The charges on ions are shown in the formula
The charges on ions are not shown in the final chemical formula. While writing the formula, the charges are used to determine the ratio of ions, but they are not written as superscripts in the final formula. For example, NaCl is written, not Na⁺Cl⁻.
Q24. The simplified formula of calcium oxide is
The simplified formula of calcium oxide is CaO. Calcium has a valency of +2 and oxygen has a valency of -2. They combine in a 1:1 ratio, and the formula is simplified to CaO. The ratio of Ca to O is 1:1.
Q25. While writing a chemical formula, the charges on ions must
While writing a chemical formula, the charges on ions must balance. The total positive charge must equal the total negative charge in a neutral compound. This is achieved by adjusting the number of ions so that the charges cancel out.
Q26. Valency helps in determining the
Valency helps in determining the formula of a compound. By knowing the valency of each element, we can determine the ratio in which atoms combine to form a compound. For example, knowing that oxygen has a valency of 2 and hydrogen has a valency of 1 helps us write the formula H₂O.
Q27. In magnesium chloride, the ratio of Mg²⁺ to Cl⁻ ions is
In magnesium chloride, the ratio of Mg²⁺ to Cl⁻ ions is 1:2. The formula is MgCl₂, which shows one magnesium ion for every two chloride ions. This balances the +2 charge of magnesium with the -1 charge of each chloride ion.
Q28. Brackets are not used in sodium carbonate because
Brackets are not used in sodium carbonate because it has one carbonate ion. The formula is Na₂CO₃, with only one carbonate ion (CO₃²⁻), so brackets are not required. Brackets are only used when there are two or more polyatomic ions.
Q29. Aluminium chloride has the formula
Aluminium chloride has the formula AlCl₃. Aluminium has a valency of +3 and chlorine has a valency of -1. Three chlorine atoms are needed to balance one aluminium ion, giving the formula AlCl₃. This is a common Lewis acid used in organic chemistry.
Q30. The formula Na₂O shows that sodium has a valency of
The formula Na₂O shows that sodium has a valency of 1. In Na₂O, two sodium atoms combine with one oxygen atom. Since oxygen has a valency of 2, each sodium must have a valency of 1 to balance the charges (2 × +1 = +2, balancing -2).
Q31. The formula of aluminium oxide is
The formula of aluminium oxide is Al₂O₃. Aluminium has a valency of +3 and oxygen has a valency of -2. The crossover method gives the formula Al₂O₃ (2 aluminium atoms for every 3 oxygen atoms). This compound is a common ceramic material.
Q32. The chemical formula of hydrogen sulphide is
The chemical formula of hydrogen sulphide is H₂S. Hydrogen has a valency of +1 and sulphur has a valency of -2. Two hydrogen atoms combine with one sulphur atom to form H₂S. This is a colourless, poisonous gas with a rotten egg smell.
Q33. The initial unsimplified formula of calcium oxide could be
The initial unsimplified formula of calcium oxide could be Ca₂O₂. Calcium has a valency of +2 and oxygen has a valency of -2. Using the crossover method, we get Ca₂O₂, which simplifies to CaO by dividing both subscripts by 2. The final formula is CaO.
Q34. Sodium oxide has the chemical formula
Sodium oxide has the chemical formula Na₂O. Sodium has a valency of +1 and oxygen has a valency of -2. Two sodium atoms combine with one oxygen atom to balance the charges, giving Na₂O. This is a white solid used in glass making.
Q35. The valency of oxygen is
The valency of oxygen is 2. Oxygen has six electrons in its valence shell and needs two more to achieve a stable octet configuration. It gains two electrons to form an O²⁻ ion, giving it a valency of 2. This is why water has the formula H₂O.
Q36. The formula of ammonium sulphate is
The formula of ammonium sulphate is (NH₄)₂SO₄. Ammonium ion (NH₄⁺) has a charge of +1 and sulphate ion (SO₄²⁻) has a charge of -2. Two ammonium ions are needed to balance one sulphate ion. The brackets indicate that ammonium is a polyatomic ion.
Q37. The chemical formula of magnesium chloride is
The chemical formula of magnesium chloride is MgCl₂. Magnesium has a valency of +2 and chlorine has a valency of -1. Two chlorine atoms are needed to balance one magnesium ion, giving the formula MgCl₂. This is a common ionic compound used in de-icing.
Q38. Which compound contains a polyatomic ion?
Ca(OH)₂ contains a polyatomic ion (OH⁻). Hydroxide is a polyatomic ion because it consists of two atoms (oxygen and hydrogen) bonded together and carries a charge. HCl, NaCl, and MgO do not contain polyatomic ions—they are binary compounds with only single atoms.
Q39. In a compound of a metal and a non-metal, the symbol written first is of the
In a compound of a metal and a non-metal, the symbol written first is of the metal. This is a general convention in writing chemical formulas. For example, in NaCl (sodium chloride), sodium (metal) is written first; in CaO (calcium oxide), calcium (metal) is written first.
Q40. Charges on ions are written while
Charges on ions are written while showing ions only. When writing the chemical formula of a compound, the charges are not included in the final formula. They are used only to determine the ratio of ions. For example, Na⁺ and Cl⁻ combine to form NaCl (without showing the charges).
Q41. Carbon tetrachloride contains how many chlorine atoms?
Carbon tetrachloride contains four chlorine atoms. Its chemical formula is CCl₄. The prefix “tetra” means four, indicating that there are four chlorine atoms bonded to one carbon atom. This is a covalent compound used as a solvent.
Q42. While writing a chemical formula, the overall charge of the compound must be
While writing a chemical formula, the overall charge of the compound must be zero. This means the total positive charge must equal the total negative charge. A chemical compound is electrically neutral overall, so the charges must balance to zero.
Q43. CaCO₃ is the chemical formula of
CaCO₃ is the chemical formula of calcium carbonate. It is formed from calcium ions (Ca²⁺) and carbonate ions (CO₃²⁻). This compound is found in limestone, chalk, and marble. It is used in construction and as a calcium supplement.
Q44. KNO₃ represents
KNO₃ represents potassium nitrate. It is formed from potassium ions (K⁺) and nitrate ions (NO₃⁻). Potassium nitrate is a common salt used in fertilizers, gunpowder, and food preservation. The nitrate ion is a polyatomic ion.
Q45. The chemical formula of sodium nitrate is
The chemical formula of sodium nitrate is NaNO₃. Sodium has a valency of +1 and nitrate ion (NO₃⁻) has a valency of -1. They combine in a 1:1 ratio to balance the charges. This compound is used in fertilizers and explosives.
Q46. Which of the following is written correctly?
NaCl is written correctly. In writing chemical formulas, the metal is written first followed by the non-metal. Sodium (Na) is a metal and chlorine (Cl) is a non-metal. The formula NaCl follows this convention. SFe, ClO, and NaCa do not follow the correct order or are not valid compounds.
Q47. Some elements show more than one valency, which is indicated by
Some elements show more than one valency, which is indicated by a Roman numeral. For example, iron shows valencies of +2 and +3, written as iron(II) and iron(III). This system helps distinguish between different compounds of the same element with different valencies.
Q48. The chemical formula of carbon tetrachloride is
The chemical formula of carbon tetrachloride is CCl₄. Carbon has a valency of 4 and chlorine has a valency of 1. Four chlorine atoms combine with one carbon atom, giving the formula CCl₄. The prefix “tetra” indicates four chlorine atoms.
Q49. The valency of sulphate ion (SO₄) is
The valency of sulphate ion (SO₄²⁻) is -2. This polyatomic ion carries a charge of -2 because it has gained two electrons. It is one of the most common anions in chemistry, forming compounds like magnesium sulphate (MgSO₄) and sodium sulphate (Na₂SO₄).
Q50. CaCl₂ is called
CaCl₂ is called calcium chloride. The metal (calcium) is written first followed by the non-metal (chlorine) with the suffix “-ide.” This is the standard naming convention for binary ionic compounds. Calcium chloride is used in de-icing roads and as a desiccant.