Practice problems on the Law of Conservation of Matter, including questions on mass calculations, chemical formulas, and reaction models.

Let's solve each problem step by step, explaining the reasoning behind each answer.

---

Problem 1:

Question: A student adds 10 grams of vinegar to 7 grams of baking soda in a sealed container. Then he watches the chemical reaction that occurs. Which of these predicts the mass of the materials in the container after the reaction is complete?
- A. 70 grams
- B. 17 grams
- C. 3 grams
- D. 0 grams

Solution:
The Law of Conservation of Mass states that matter cannot be created or destroyed in a closed system. The total mass of reactants must equal the total mass of products.

- Initial mass of vinegar = 10 grams
- Initial mass of baking soda = 7 grams
- Total initial mass = 10 + 7 = 17 grams

During the reaction, vinegar (acetic acid) reacts with baking soda (sodium bicarbonate) to produce carbon dioxide gas, water, and sodium acetate. Since the container is sealed, no mass is lost to the surroundings. Therefore, the total mass of the materials in the container after the reaction will still be 17 grams.

Answer: B. 17 grams

---

Problem 2:

Question: Which procedure best represents the law of conservation of mass?
- a. evaporating salt water leaves 5 grams of salt
- b. burning 2 kilograms of wood leaves 0.4 kilogram of ash
- c. using filter paper to separate 2 grams of solid from 10 grams of water
- d. combining 2 grams of copper and 4 grams of sulfur to produce 6 grams of copper sulfate

Solution:
The Law of Conservation of Mass states that the total mass of reactants must equal the total mass of products in a closed system.

- Option a: Evaporating salt water leaves 5 grams of salt. This does not conserve mass because the water vapor is lost to the surroundings.
- Option b: Burning 2 kilograms of wood leaves 0.4 kilogram of ash. This does not conserve mass because the rest of the mass is released as gases (e.g., CO₂ and H₂O).
- Option c: Using filter paper to separate 2 grams of solid from 10 grams of water. This is a physical separation, not a chemical reaction, and it does not demonstrate the conservation of mass.
- Option d: Combining 2 grams of copper and 4 grams of sulfur to produce 6 grams of copper sulfate. Here, the total mass of reactants (2 + 4 = 6 grams) equals the total mass of the product (6 grams). This demonstrates the Law of Conservation of Mass.

Answer: d. combining 2 grams of copper and 4 grams of sulfur to produce 6 grams of copper sulfate

---

Problem 3:

Question: In the chemical formula for Magnesium chloride, NaCl₄, what does the subscript 4 represent?
- a. 2 molecules of Magnesium
- b. Nothing
- c. 2 molecules of Chlorine
- d. 4 molecules of Chlorine

Solution:
The chemical formula NaCl₄ is incorrect for magnesium chloride. The correct formula for magnesium chloride is MgCl₂. However, assuming the question is about the subscript in general:

- The subscript in a chemical formula indicates the number of atoms of that element in one molecule.
- In MgCl₂, the subscript 2 means there are 2 chlorine atoms per molecule of magnesium chloride.
- If we were to consider NaCl₄ hypothetically, the subscript 4 would mean there are 4 chlorine atoms per molecule.

Since the question asks about NaCl₄, the subscript 4 represents 4 chlorine atoms.

Answer: d. 4 molecules of Chlorine

---

Problem 4:

Question: Which model demonstrates the Law of Conservation of Matter?

[Models A, B, C, D are shown in the image]

Solution:
The Law of Conservation of Matter states that the total number of atoms of each element must be the same before and after a chemical reaction.

- Model A: Shows 1 atom of one type and 1 atom of another type reacting to form 1 molecule. The number of atoms is conserved.
- Model B: Shows 1 atom of one type and 2 atoms of another type reacting to form 1 molecule. The number of atoms is not conserved.
- Model C: Shows 2 atoms of one type and 2 atoms of another type reacting to form 2 molecules. The number of atoms is conserved.
- Model D: Shows 2 atoms of one type and 2 atoms of another type reacting to form 1 molecule. The number of atoms is not conserved.

Both Models A and C conserve the number of atoms, but Model A is simpler and directly shows the conservation of matter.

Answer: A

---

Problem 5:

Question: Which sample equation best illustrates the law of conservation of mass?
- a. 8 grams reactant U + 3 grams reactant V → 5 grams product W
- b. 12 grams reactant X + 10 grams reactant Y → 22 grams product Z
- c. 5 grams reactant E + 5 grams reactant F → 20 grams product G + 5 grams product H
- d. 10 grams reactant Q + 9 grams reactant R → 90 grams product S + 90 grams product T

Solution:
The Law of Conservation of Mass states that the total mass of reactants must equal the total mass of products.

- Option a: 8 + 3 = 11 grams (reactants) ≠ 5 grams (products)
- Option b: 12 + 10 = 22 grams (reactants) = 22 grams (products)
- Option c: 5 + 5 = 10 grams (reactants) ≠ 20 + 5 = 25 grams (products)
- Option d: 10 + 9 = 19 grams (reactants) ≠ 90 + 90 = 180 grams (products)

Only Option b conserves mass.

Answer: b. 12 grams reactant X + 10 grams reactant Y → 22 grams product Z

---

Problem 6:

Question: The students placed an antacid tablet in a ziplock bag. The mass of the tablet, 30 grams, and the bag, 40 grams. Then they carefully added 30 grams of water and quickly sealed the bag. The tablet began to fizz and soon disappeared. The bag was filled with gas. If the total mass of the bag + tablet + water was 130 grams before the reaction, what was the total mass of the bag + liquid + gas after the reaction was completed?
- A. 70 grams
- B. 30 grams
- C. 130 grams
- D. 160 grams

Solution:
The Law of Conservation of Mass states that the total mass of the system remains constant if it is closed. The system here includes the bag, tablet, water, and any gases produced.

- Initial total mass = mass of bag + mass of tablet + mass of water = 40 + 30 + 30 = 130 grams
- After the reaction, the tablet disappears, but the mass of the system (bag + liquid + gas) must still be 130 grams because no mass is lost or gained.

Answer: C. 130 grams

---

Problem 7:

Question: A cube of water measured at 200 grams. The cup of water was placed in the freezer. What is the mass of the frozen cup of water?
- A. 250 grams
- B. 150 grams
- C. 200 grams
- D. 300 grams

Solution:
The Law of Conservation of Mass states that the mass of a substance does not change during a phase change (e.g., liquid to solid). When water freezes, it changes from a liquid to a solid, but its mass remains the same.

- Initial mass of water = 200 grams
- Mass of frozen water = 200 grams

Answer: C. 200 grams

---

Final Answers:

1. B. 17 grams
2. d. combining 2 grams of copper and 4 grams of sulfur to produce 6 grams of copper sulfate
3. d. 4 molecules of Chlorine
4. A
5. b. 12 grams reactant X + 10 grams reactant Y → 22 grams product Z
6. C. 130 grams
7. C. 200 grams

Boxed Final Answer:
\[
\boxed{B, d, d, A, b, C, C}
\]