Question

1. choose the best answer.

which reaction between phosphorus trihydride and oxygen follows the law of conservation of mass?
4ph₃ + o₂ → p₄h₁₀ + 2h₂o
4ph₃ + o₂ → p₄h₁₀ + 6h₂o
8ph₃ + o₂ → 2p₄h₁₀ + 2h₂o
8ph₃ + 2o₂ → 2p₄h₁₀ + 2h₂o

1. choose the best answer.

what is the balanced equation for the combustion of magnesium?
mg(s) + o₂(g) → mgo(g)
mg(s) + 2o₂(g) → mgo₄²⁻(aq)
2mg(s) + o₂(g) → mgo₂(g)
2mg(s) + o₂(g) → 2mgo(s)

Explanation:

Question 4

Step1: Check P atoms

For each option, count P atoms on left (reactants) and right (products).

• Option 1: Left: \(4\) (from \(4\text{PH}_3\)), Right: \(4\) (from \(\text{P}_4\text{H}_{10}\)) → P balanced.
• Option 2: Left: \(4\), Right: \(4\) → P balanced.
• Option 3: Left: \(8\), Right: \(2\times4 = 8\) → P balanced.
• Option 4: Left: \(8\), Right: \(2\times4 = 8\) → P balanced.

Step2: Check H atoms

Count H atoms.

• Option 1: Left: \(4\times3 = 12\), Right: \(10 + 2\times2 = 14\) → Not balanced.
• Option 2: Left: \(4\times3 = 12\), Right: \(10 + 6\times2 = 22\) → Not balanced.
• Option 3: Left: \(8\times3 = 24\), Right: \(2\times10 + 2\times2 = 24\) → H balanced. Now check O. Left: \(2\) (from \(\text{O}_2\)), Right: \(2\times1 = 2\) (from \(2\text{H}_2\text{O}\)) → O balanced? Wait, no, wait. Wait, option 3: \(8\text{PH}_3 + \text{O}_2 \to 2\text{P}_4\text{H}_{10} + 2\text{H}_2\text{O}\). H: 24 vs 24. O: Left 2, Right 2 (from \(2\text{H}_2\text{O}\), each has 1 O? Wait no, \(2\text{H}_2\text{O}\) has 2 O? Wait no, \( \text{H}_2\text{O}\) has 1 O per molecule. So \(2\text{H}_2\text{O}\) has 2 O. Left O: 2 (from \(\text{O}_2\)). So O balanced? Wait but option 4: \(8\text{PH}_3 + 2\text{O}_2 \to 2\text{P}_4\text{H}_{10} + 2\text{H}_2\text{O}\). H: \(8\times3 = 24\), Right: \(2\times10 + 2\times2 = 24\) → H balanced. O: Left: \(2\times2 = 4\), Right: \(2\times1 = 2\) → No. Wait, wait I made a mistake. Let's recheck option 3: \(8\text{PH}_3 + \text{O}_2 \to 2\text{P}_4\text{H}_{10} + 2\text{H}_2\text{O}\). O: Left 2, Right 2 (from \(2\text{H}_2\text{O}\), each has 1 O? No, \( \text{H}_2\text{O}\) has 1 O, so 2 molecules have 2 O. Left O: 2. So O balanced? Wait but option 4: \(8\text{PH}_3 + 2\text{O}_2 \to 2\text{P}_4\text{H}_{10} + 2\text{H}_2\text{O}\). O: Left 4, Right 2 → Not balanced. Wait, option 3: H is balanced (24), O: 2 vs 2. Wait, but let's check option 4 again. Wait, no, wait the correct approach: the law of conservation of mass requires balanced chemical equations (same number of each atom on both sides). Let's re-express:

Wait, the correct answer is option 4? Wait no, wait let's do option 4: \(8\text{PH}_3 + 2\text{O}_2 \to 2\text{P}_4\text{H}_{10} + 2\text{H}_2\text{O}\). P: 8 vs 8. H: 24 vs 24. O: 4 vs 2. No. Wait option 3: O: 2 vs 2. Wait, but let's check again. Wait, maybe I messed up. Wait the correct answer is the last option? Wait no, wait let's check the fourth option: \(8\text{PH}_3 + 2\text{O}_2 \to 2\text{P}_4\text{H}_{10} + 2\text{H}_2\text{O}\). Wait, no, let's check H again. \(8\text{PH}_3\) has \(8\times3 = 24\) H. \(2\text{P}_4\text{H}_{10}\) has \(2\times10 = 20\) H, \(2\text{H}_2\text{O}\) has \(2\times2 = 4\) H. Total 24. O: \(2\text{O}_2\) has 4 O, \(2\text{H}_2\text{O}\) has 2 O. No. Wait option 3: \(8\text{PH}_3 + \text{O}_2 \to 2\text{P}_4\text{H}_{10} + 2\text{H}_2\text{O}\). O: 2 vs 2. H: 24 vs 24. P: 8 vs 8. So that's balanced? Wait but the fourth option: \(8\text{PH}_3 + 2\text{O}_2 \to 2\text{P}_4\text{H}_{10} + 2\text{H}_2\text{O}\). O: 4 vs 2. No. Wait, maybe the correct answer is the fourth option? Wait no, I think I made a mistake. Wait let's check the original options again. Wait the fourth option is \(8\text{PH}_3 + 2\text{O}_2 \to 2\text{P}_4\text{H}_{10} + 2\text{H}_2\text{O}\). Let's count O: Left 4 (2 O₂), Right 2 (2 H₂O). No. Option 3: O: 2 vs 2. So option 3? Wait but the user's options: let's re-express:

Option 1: \(4\text{PH}_3 + \text{O}_2 \to \text{P}_4\text{H}_{10} + 2\text{H}_2\text{O}\). H: 12 vs 14. No.

Option 2: \(4\text{PH}_3 + \text{O}_2 \to \text{P}_4\text{H}_{10…

Step1: Recall combustion of Mg

Magnesium burns in oxygen to form magnesium oxide (MgO), solid. The reaction is \( \text{Mg}(s) + \text{O}_2(g) \to \text{MgO}(s) \), but this is unbalanced.

Step2: Balance the equation

Balance Mg and O.

• Mg: Left 1, Right 1. O: Left 2, Right 1. So multiply MgO by 2, and Mg by 2.
• Balanced equation: \( 2\text{Mg}(s) + \text{O}_2(g) \to 2\text{MgO}(s) \).

Check options:

• Option 1: Unbalanced (O: 2 vs 1).
• Option 2: Incorrect product (MgO₄²⁻ is not formed in combustion).
• Option 3: Incorrect product (MgO₂ is not the product).
• Option 4: \(2\text{Mg}(s) + \text{O}_2(g) \to 2\text{MgO}(s)\) → Balanced.

Answer:

(Question 4):
The balanced equation (following law of conservation of mass) is \( \boldsymbol{8\text{PH}_3 + \text{O}_2 \to 2\text{P}_4\text{H}_{10} + 2\text{H}_2\text{O}} \) (the third option: \(8\text{PH}_3 + \text{O}_2 \to 2\text{P}_4\text{H}_{10} + 2\text{H}_2\text{O}\))