High School

13. Mariah mixes in a large bowl 400 g of flour with 15 g of water and 40 g of butter to make dough. He then takes out 20 g to test. How much dough is left in his bowl?

A. 415 grams
B. 425 grams
C. 435 grams
D. 445 grams

14. Applying the Law of Conservation of Mass, what should be the total mass of the product in the chemical equation below? (H = 1g; O = 16 g)

2H₂ + O₂ → 2H₂O

A. 17 grams
B. 18 grams
C. 36 grams
D. 72 grams

15. Which chemical equation supports the law of conservation of mass?

A. 2H₂O (l) → H₂ (g) + O₂ (g)
B. Zn (s) + HCl (aq) → ZnCl₂(aq) + H₂ (g)
C. Al₄C₃ (s) + H₂O (l) → CH₄ (g) + Al(OH)₃ (s)
D. CH₄ (g) + 2O₂ (g) → CO₂ (g) + 2H₂O (g)

Answer :

Let's go through each question step-by-step.

  1. The question asks how much dough is left after Mariah takes out 20 g to test.

  • Start by calculating the total mass of the dough mixed initially. Mariah uses 400 g of flour, 15 g of water, and 40 g of butter.

    [tex]\text{Total initial mass} = 400 \text{ g} + 15 \text{ g} + 40 \text{ g} = 455 \text{ g}[/tex]

  • Then, 20 g of dough is taken out for testing.

    [tex]\text{Dough left} = 455 \text{ g} - 20 \text{ g} = 435 \text{ g}[/tex]

Therefore, the amount of dough left in the bowl is 435 grams.

The correct answer is C. 435 grams.

  1. Applying the Law of Conservation of Mass involves stating that mass is neither created nor destroyed in a chemical reaction. So, the total mass before the reaction must equal the total mass after it.

  • In the equation [tex]2\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O}[/tex], let's calculate the mass of the reactants first.

    • Two molecules of hydrogen [tex](\text{H}_2)[/tex] equals 4 g since each molecule [tex](\text{H})[/tex] is 1 g:
      [tex]2 \times (2 \times 1 \text{ g}) = 4 \text{ g}[/tex]

    • One molecule of oxygen [tex](\text{O}_2)[/tex] equals 32 g since [tex]\text{O}_2[/tex] is 16 g per atom:
      [tex]32 \text{ g}[/tex]

  • Total mass of reactants = 4 g + 32 g = 36 g.

  • Using the law, total mass of products is also 36 g.

Therefore, the total mass of the product is C. 36 grams.

  1. To support the law of conservation of mass, the number of atoms for each element must be the same on both sides of the equation.

  • For option A, [tex]2\text{H}_2\text{O} (l) \rightarrow \text{H}_2 (g) + \text{O}_2 (g)[/tex]: The mass is balanced but the number of atoms is not equal on both sides.

  • For option B, [tex]\text{Zn (s)} + \text{HCl (aq)} \rightarrow \text{ZnCl}_2 (aq) + \text{H}_2 (g)[/tex]: The equation is not balanced as written.

  • For option C, [tex]\text{Al}_4\text{C}_3 (s) + \text{H}_2\text{O} (l) \rightarrow \text{CH}_4 (g) + \text{Al(OH)}_3 (s)[/tex]: It is not balanced.

  • For option D, [tex]\text{CH}_4 (g) + 2\text{O}_2 (g) \rightarrow \text{CO}_2 (g) + 2\text{H}_2\text{O} (g)[/tex]: This equation is balanced, with the same number of each type of atom on both sides.

Therefore, option D. CH₄ (g) + 2O₂ (g) → CO₂ (g) + 2H₂O (g) supports the law of conservation of mass.

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