Questions on Percent of Water in a Hydrate
Multiple-Choice Questions: Percent of Water in a Hydrate
1. What does the term "hydrate" refer to in chemistry?
A) A compound dissolved in water
B) A compound containing hydroxide ions
C) A compound that contains water molecules in its crystal structure
D) A compound formed from dehydration
E) A mixture of water and alcohol
2. What is the general formula for a hydrate?
A) MX + H₂O
B) MX + nH₂O
C) MX · nH₂O
D) MX(H₂O)n
E) M(OH)ₙ
3. In CuSO₄·5H₂O, how many water molecules are present per formula unit?
A) 1
B) 2
C) 4
D) 5
E) 6
4. What is the formula to calculate the percent of water in a hydrate?
A) (mass of water / mass of anhydrous salt) × 100
B) (mass of hydrate / mass of water) × 100
C) (mass of water / mass of hydrate) × 100
D) (mass of water / molar mass of water) × 100
E) (mass of salt / mass of hydrate) × 100
5. What is the percent of water in CuSO₄·5H₂O? (Cu=63.5, S=32, O=16, H=1)
A) 18.0%
B) 25.0%
C) 36.0%
D) 45.0%
E) 55.0%
6. What happens when a hydrate is heated?
A) It becomes a gas
B) It decomposes into elements
C) It loses its water of hydration
D) It reacts with oxygen
E) It becomes more hydrated
7. Which of the following is a hydrate?
A) NaCl
B) MgSO₄
C) CaCl₂·2H₂O
D) CO₂
E) HCl
8. The percent by mass of water in BaCl₂·2H₂O is closest to:
A) 8.6%
B) 14.7%
C) 18.3%
D) 24.2%
E) 30.0%
9. Which of the following hydrates contains the most water by percent mass?
A) CuSO₄·5H₂O
B) Na₂CO₃·10H₂O
C) MgSO₄·7H₂O
D) BaCl₂·2H₂O
E) CoCl₂·6H₂O
10. A hydrate has a molar mass of 250 g/mol, and 90 g of that is water. What is the percent of water?
A) 20%
B) 30%
C) 36%
D) 40%
E) 45%
11. If a hydrate weighs 180 g and becomes 120 g after heating, what mass was lost?
A) 30 g
B) 40 g
C) 50 g
D) 60 g
E) 70 g
12. Based on the previous question, what is the percent of water in the hydrate?
A) 20%
B) 25%
C) 30%
D) 33.3%
E) 40%
13. Which term refers to a compound after all water is removed?
A) Solvate
B) Anhydrous
C) Hydrolyzed
D) Isomeric
E) Desiccated
14. What laboratory method is commonly used to determine percent water in a hydrate?
A) Filtration
B) Chromatography
C) Evaporation
D) Heating and weighing
E) Titration
15. In Na₂CO₃·10H₂O, what is the mass of water? (H=1, O=16)
A) 90 g
B) 100 g
C) 120 g
D) 130 g
E) 150 g
16. What is the molar mass of H₂O?
A) 16 g/mol
B) 17 g/mol
C) 18 g/mol
D) 19 g/mol
E) 20 g/mol
17. The percent water in Na₂CO₃·10H₂O is approximately:
A) 30%
B) 40%
C) 50%
D) 60%
E) 70%
18. Which factor affects the accuracy of water percent determination in a hydrate experiment?
A) Color of compound
B) Type of balance used
C) Purity of hydrate
D) All of the above
E) None of the above
19. After heating a hydrate, the remaining compound is called:
A) Water of hydration
B) Base compound
C) Solute
D) Anhydrous salt
E) Precipitate
20. The purpose of using a crucible in hydrate experiments is to:
A) Melt the compound
B) Prevent evaporation
C) Weigh the compound
D) Contain and heat the sample safely
E) Dissolve the hydrate
Answer Key with Explanations
1. C – A hydrate contains water molecules bound in its structure.
2. C – The correct representation is: MX · nH₂O
3. D – The "5" indicates 5 water molecules per unit.
4. C – % H₂O = (mass of water / mass of hydrate) × 100
5. B – CuSO₄·5H₂O = 5×18 = 90 g water; total molar mass ≈ 250 → 90/250 = 36%
6. C – Heating drives off the water of hydration.
7. C – Only CaCl₂·2H₂O is a hydrate.
8. A – BaCl₂·2H₂O ≈ 244 g/mol; 2×18 = 36 g → 36/244 ≈ 14.7%
9. B – Na₂CO₃·10H₂O has a high proportion of water (~63%).
10. D – 90/250 = 36%
11. D – 180 - 120 = 60 g
12. D – 60/180 × 100 = 33.3%
13. B – Anhydrous = "without water"
14. D – Heating and weighing is standard in hydrate labs.
15. B – 10 × 18 = 180 g/mol water
16. C – 2(1) + 16 = 18 g/mol
17. C – 180/286 ≈ 63%
18. D – All listed factors can affect results.
19. D – What remains is the anhydrous salt.
20. D – Crucibles are heat-resistant and used for high-temperature heating.
