“Stoichiometry” MCQ with Answer Explanation.

21. In a reaction where 2 moles of A react with 3 moles of B to produce 4 moles of C, if 1.5 moles of A are used, how many moles of C are produced?

A) 2 moles
B) 3 moles
C) 4 moles
D) 6 moles

Answer: B) 3 moles

Explanation: The ratio of A to C is 2:4, or 1:2. Therefore, 1.5 moles of A produce 1.5 × 2 = 3 moles of C.

22. How many grams of H₂ are produced from the electrolysis of water? If 18 g of water are decomposed. (Molar mass of H₂ = 2 g/mol)

A) 4 g
B) 6 g
C) 8 g
D) 12 g

Answer: B) 6 g

Explanation: 18 g of water equals 1 mole (18 g / 18 g/mol). From the decomposition of water, 1 mole of water produces 2 moles of H₂, which equals 2 × 2 g/mol = 4 g.

23. In the reaction 2Mg + O₂ → 2MgO, how many grams of magnesium oxide (MgO) can be produced from 3 moles of magnesium? (Molar mass of MgO = 40 g/mol)

A) 40 g
B) 60 g
C) 80 g
D) 120 g

Answer: C) 120 g

Explanation: From the equation, 2 moles of Mg produce 2 moles of MgO. Therefore, 3 moles of Mg will produce 3 moles of MgO. Mass = 3 moles × 40 g/mol = 120 g.

24. What is the number of molecules in 1 mole of a substance?

A) 6.022 × 10²²
B) 6.022 × 10²³
C) 6.022 × 10²⁴
D) 6.022 × 10²⁵

Answer: B) 6.022 × 10²³

Explanation: Avogadro’s number states that 1 mole of any substance contains 6.022 × 10²³ molecules.

25. If you have 4 moles of H₂ and 2 moles of O₂, how many moles of water (H₂O) can be produced?

A) 2 moles
B) 4 moles
C) 6 moles
D) 8 moles

Answer: B) 4 moles

Explanation: The reaction is 2H₂ + O₂ → 2H₂O. Therefore, 4 moles of H₂ can produce 4 moles of H₂O with 2 moles of O₂.

26. In the reaction C + O₂ → CO₂, how many grams of carbon are needed to produce 88 g of CO₂? (Molar mass of CO₂ = 44 g/mol, C = 12 g/mol)

A) 12 g
B) 24 g
C) 36 g
D) 48 g

Answer: B) 24 g

Explanation: 88 g of CO₂ equals 2 moles (88 g / 44 g/mol). From the balanced equation, 1 mole of C produces 1 mole of CO₂, so 2 moles of C are needed, which equals 2 × 12 g/mol = 24 g.

27. What is the mass of 5 moles of sulfuric acid (H₂SO₄)? (Molar mass of H₂SO₄ = 98 g/mol)

A) 490 g
B) 500 g
C) 540 g
D) 560 g

Answer: A) 490 g

Explanation: Mass = moles × molar mass = 5 moles × 98 g/mol = 490 g.

28. In the equation 2Na + Cl₂ → 2NaCl, what is the mass of sodium chloride produced from 2 moles of sodium? (Molar mass of NaCl = 58.5 g/mol)

A) 58.5 g
B) 117 g
C) 234 g
D) 350 g

Answer: B) 117 g

Explanation: 2 moles of Na produce 2 moles of NaCl. Therefore, the mass of NaCl = 2 × 58.5 g/mol = 117 g.

29. How many moles of NaOH are needed to neutralize 1 mole of HCl?

A) 0.5 moles
B) 1 mole
C) 2 moles
D) 3 moles

Answer: B) 1 mole

Explanation: The neutralization reaction is HCl + NaOH → NaCl + H₂O, which shows a 1:1 ratio.

30. If 0.5 moles of Al react with O₂, how many moles of Al₂O₃ are formed? (4Al + 3O₂ → 2Al₂O₃)

A) 0.25 moles
B) 0.5 moles
C) 1 mole
D) 2 moles

Answer: A) 0.25 moles

Explanation: The reaction shows that 4 moles of Al produce 2 moles of Al₂O₃. Thus, 0.5 moles of Al will produce 0.5 × (2/4) = 0.25 moles of Al₂O₃.