What is the formula of iron(III) oxide?
A) FeO
B) Fe3O4
C) FeO2
D) Fe2O3
▶️ Answer/Explanation
Ans: D
Iron(III) oxide means iron has an oxidation state of +3 (Fe³⁺) and oxygen has -2 (O²⁻). To balance the charges, we need two Fe³⁺ ions (total +6 charge) and three O²⁻ ions (total -6 charge), giving the formula Fe2O3.
Option A (FeO) is iron(II) oxide. Option B (Fe3O4) is a mixed oxide containing both Fe²⁺ and Fe³⁺. Option C (FeO2) is not a common iron oxide.
What is the empirical formula of a compound that contains 3.66 g of hydrogen, 37.8 g of phosphorus and 58.5 g of oxygen?
A) H6P2O6
B) H4PO4
C) H3PO3
D) HPO
▶️ Answer/Explanation
Ans: C
To find the empirical formula:
1. Convert masses to moles:
- Hydrogen: 3.66 g ÷ 1 g/mol = 3.66 mol
- Phosphorus: 37.8 g ÷ 31 g/mol ≈ 1.22 mol
- Oxygen: 58.5 g ÷ 16 g/mol ≈ 3.66 mol
2. Divide by smallest number of moles (1.22):
- H: 3.66 ÷ 1.22 ≈ 3
- P: 1.22 ÷ 1.22 = 1
- O: 3.66 ÷ 1.22 ≈ 3
3. This gives the ratio H3PO3, which matches option C.
Ammonium iron(III) citrate contains in its formula:
- more than one ammonium ion
- one iron ion
- two C6H4O74- ions.
What is the formula of ammonium iron(III) citrate?
A) (NH4)4Fe(C6H4O7)2
B) (NH4)5Fe(C6H4O7)2
C) (NH4)6Fe(C6H4O7)2
D) (NH4)7Fe(C6H4O7)2
▶️ Answer/Explanation
Ans: B
Each citrate ion (C6H4O74-) carries 4- charge. With two citrate ions, total negative charge is 8-. Iron(III) contributes 3+ charge. To balance, we need 5 NH4+ ions (5 × 1+ = 5+). Total positive charge: 5+ (ammonium) + 3+ (iron) = 8+, balancing the 8- from citrate. Therefore, the correct formula is (NH4)5Fe(C6H4O7)2 (option B).
Silicon(IV) oxide reacts with chlorine and carbon to form liquid silicon(IV) chloride, SiCl4, and carbon dioxide gas.
If the reaction is carried out at r.t.p., which symbol equation represents this reaction?
A) SiO2(l) + 2Cl2(g) + C(s) → SiCl4(l) + CO2(g)
B) SiO2(l) + 2Cl2(g) + C(g) → SiCl4(l) + CO2(g)
C) SiO2(s) + 2Cl2(g) + C(s) → SiCl4(g) + CO2(g)
D) SiO2(s) + 2Cl2(g) + C(s) → SiCl4(l) + CO2(g)
▶️ Answer/Explanation
Ans: D
The correct equation must account for the physical states: SiO2 is solid (s), Cl2 is gas (g), carbon is solid (s), SiCl4 is liquid (l) at r.t.p., and CO2 is gas (g). Therefore, option D is correct. Option A incorrectly shows SiO2 as liquid, B shows carbon as gas (unlikely at r.t.p.), and C shows SiCl4 as gas when it should be liquid.