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Empirical & Chemical Formulas: MCQs and Short Questions for Chemistry Practice

 

Empirical Formula

The empirical formula of a compound gives the simplest whole-number ratio of atoms of each element present in the compound. It does not provide information about the actual number of atoms or molecular structure, only the proportional ratio.

How to Determine the Empirical Formula?

  1. Find the mass percentage of each element in the compound.
  2. Convert the mass to moles by dividing by atomic masses.
  3. Divide each by the smallest number of moles to get a whole-number ratio.

Example:

  • Glucose (C₆H₁₂O₆) → Its empirical formula is CH₂O because the ratio of carbon, hydrogen, and oxygen is 1:2:1.
  • Benzene (C₆H₆) → Its empirical formula is CH, as the ratio of carbon to hydrogen is 1:1.

Key Point:

The empirical formula is useful in identifying unknown compounds and is determined through experimental data.


Chemical Formula

A chemical formula shows the actual number of atoms of each element in a molecule. It provides complete information about the atomic composition of a compound.

Types of Chemical Formulas:

  1. Molecular Formula – Shows the actual number of atoms in a molecule. (e.g., C₂H₆ for ethane)
  2. Structural Formula – Represents how atoms are arranged in a molecule.
  3. Empirical Formula – Shows only the simplest ratio of atoms.

Examples:

  • Water (H₂O): Contains 2 hydrogen and 1 oxygen atom.
  • Methane (CH₄): Contains 1 carbon and 4 hydrogen atoms.
  • Sulfuric Acid (H₂SO₄): Contains 2 hydrogen, 1 sulfur, and 4 oxygen atoms.

Key Point:

Chemical formulas help chemists understand the composition and behavior of substances in chemical reactions.


Chemical Formula of a Compound

A compound is a pure substance composed of two or more elements chemically bonded together. The chemical formula of a compound shows the exact number of each type of atom in a molecule of that compound.

Examples of Chemical Formulas for Common Compounds:

  • Ammonia (NH₃): Contains one nitrogen and three hydrogen atoms.
  • Carbon Dioxide (CO₂): Contains one carbon and two oxygen atoms.
  • Glucose (C₆H₁₂O₆): Contains six carbon, twelve hydrogen, and six oxygen atoms.

Why is the Chemical Formula Important?

  • It helps in writing balanced chemical equations.
  • It provides details about molecular composition.
  • It helps predict chemical properties of a substance.

Chemical Formula of a Binary Ionic Compound

A binary ionic compound is a compound composed of two different elements:

  1. A metal (cation, positively charged ion).
  2. A non-metal (anion, negatively charged ion).

Rules for Writing the Formula of a Binary Ionic Compound:

  1. Write the symbol of the cation (metal) first.
  2. Write the symbol of the anion (non-metal) second.
  3. Balance the charges using subscripts so that the total positive and negative charges are equal.

Examples:

Explanation:

  • In NaCl, Na⁺ has +1 charge, and Cl⁻ has -1 charge, so they combine in a 1:1 ratio.
  • In CaF₂, Ca²⁺ has +2 charge, and F⁻ has -1 charge, so two fluoride ions are needed to balance one calcium ion.

Key Point:

The chemical formula of a binary ionic compound is determined by charge balancing between cations and anions.

Here are 20 MCQs and 20 short questions based on empirical formulas, chemical formulas, and binary ionic compounds for your website ChemCore9-10.


Multiple Choice Questions (MCQs)

1. The empirical formula represents:
A) The actual number of atoms in a compound
B) The simplest whole-number ratio of atoms
C) The structural arrangement of atoms
D) The molecular weight of a compound
(Answer: B)

2. The empirical formula of glucose (C₆H₁₂O₆) is:
A) C₆H₁₂O₆
B) CH₂O
C) C₂H₄O₂
D) CO₂
(Answer: B)

3. The chemical formula of water is:
A) HO
B) H₂O₂
C) H₂O
D) OH
(Answer: C)

4. Which of the following is a binary ionic compound?
A) H₂O
B) CO₂
C) NaCl
D) C₆H₁₂O₆
(Answer: C)

5. A compound with an empirical formula of CH₂ has a molecular mass of 56 g/mol. What is its molecular formula?
A) CH₂
B) C₂H₄
C) C₄H₈
D) C₄H₁₀
(Answer: C)

6. The charge on the sodium ion in NaCl is:
A) -1
B) +1
C) +2
D) -2
(Answer: B)

7. The empirical formula of benzene (C₆H₆) is:
A) CH
B) C₂H₂
C) C₆H₆
D) C₃H₃
(Answer: A)

8. The total number of atoms in one molecule of H₂SO₄ is:
A) 5
B) 6
C) 7
D) 8
(Answer: D)

9. Which of the following compounds has the same empirical and molecular formula?
A) C₆H₁₂O₆
B) H₂O
C) C₂H₄
D) C₄H₈
(Answer: B)

10. The chemical formula of aluminum oxide is:
A) AlO
B) Al₂O₃
C) Al₃O₂
D) AlO₂
(Answer: B)

11. The empirical formula of a compound with 40% sulfur and 60% oxygen is:
A) SO
B) SO₂
C) S₂O₄
D) S₃O₆
(Answer: B)

12. Which of the following is not an ionic compound?
A) NaCl
B) MgO
C) CO₂
D) CaF₂
(Answer: C)

13. What is the formula of magnesium chloride?
A) MgCl
B) Mg₂Cl
C) MgCl₂
D) Mg₂Cl₂
(Answer: C)

14. The chemical formula of methane is:
A) CH₄
B) C₂H₆
C) C₃H₈
D) CH₃
(Answer: A)

15. What is the ratio of hydrogen to oxygen in the empirical formula of H₂O₂?
A) 1:1
B) 2:2
C) 2:1
D) 1:2
(Answer: C)

16. Which element is always found in binary ionic compounds?
A) Carbon
B) Hydrogen
C) A metal
D) A noble gas
(Answer: C)

17. What is the correct formula for calcium fluoride?
A) CaF
B) CaF₂
C) Ca₂F
D) Ca₂F₂
(Answer: B)

18. The empirical formula of ethene (C₂H₄) is:
A) C₄H₈
B) C₂H₄
C) CH₂
D) CH₄
(Answer: C)

19. The molecular formula of a compound is always:
A) The same as the empirical formula
B) A multiple of the empirical formula
C) More simplified than the empirical formula
D) Unrelated to the empirical formula
(Answer: B)

20. What is the formula for potassium oxide?
A) K₂O
B) KO₂
C) K₂O₂
D) KO
(Answer: A)


Short Questions

1. Define an empirical formula with an example.
2. How is the chemical formula different from the empirical formula?
3. What information does a chemical formula provide?
4. Explain why NaCl is called a binary ionic compound.
5. What is the empirical formula of C₄H₁₀?
6. Calculate the empirical formula of a compound that contains 50% sulfur and 50% oxygen.
7. What is the molecular formula of a compound with an empirical formula CH₂ and a molecular mass of 84 g/mol?
8. Why is CO₂ not an ionic compound?
9. Write the empirical formula for H₂O₂.
10. How do you determine the formula of a binary ionic compound?
11. Give an example of a compound where the empirical and molecular formulas are the same.
12. What is the empirical formula of a compound containing 80% carbon and 20% hydrogen?
13. What is the formula of magnesium oxide?
14. Write the chemical formula of calcium chloride and explain how it is formed.
15. Why must the charges of ions in an ionic compound be balanced?
16. How is the formula of Al₂O₃ determined?
17. Differentiate between ionic and covalent compounds.
18. Give an example of a binary molecular compound.
19. Why does sodium form Na⁺ instead of Na²⁺?
20. What is the importance of an empirical formula in chemistry?


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