Questions 15-19 are based on the information in the table below. [VISUAL] Which of the following is — Stoichiometry Chemistry Question

STEPS:

1. Identify the balanced skeleton equation for the combustion reaction:
According to the data table, the combustion of one mole of the unknown compound in the presence of oxygen gas is represented by the following chemical equation:
$$\text{Unknown} + 5\ \text{O}_2(g) \rightarrow 4\ \text{CO}_2(g) + 4\ \text{H}_2\text{O}(l)$$

2. Count the total number of atoms of each element on the product (right) side:
* Carbon (C): There are 4 moles of $\text{CO}_2$, which equals $4 \times 1 = \mathbf{4\text{ carbon atoms}}$.
* Hydrogen (H): There are 4 moles of $\text{H}_2\text{O}$, which equals $4 \times 2 = \mathbf{8\text{ hydrogen atoms}}$.
* Oxygen (O): Oxygen is present in both products. 4 moles of $\text{CO}_2$ contribute $4 \times 2 = 8$ oxygen atoms, and 4 moles of $\text{H}_2\text{O}$ contribute $4 \times 1 = 4$ oxygen atoms, yielding a total of $8 + 4 = \mathbf{12\text{ oxygen atoms}}$.

3. Apply the Law of Conservation of Mass:
The Law of Conservation of Mass dictates that atoms are neither created nor destroyed during a chemical reaction. Therefore, the reactant (left) side of the equation must contain exactly the same number of carbon, hydrogen, and oxygen atoms as the product side ($4\text{ C}$, $8\text{ H}$, and $12\text{ O}$).

4. Account for the atoms contributed by the known reactant, $\text{O}_2$:
The reactant side contains 5 moles of diatomic oxygen gas ($5\ \text{O}_2$), which supplies $5 \times 2 = \mathbf{10\text{ oxygen atoms}}$ to the reactants.

5. Deduct the known reactant atoms to solve for the unknown formula:
Subtract the atoms provided by $5\ \text{O}_2$ from the total atoms required on the reactant side:
* Carbon atoms needed: $4 - 0 = \mathbf{4\text{ carbon atoms}}$
* Hydrogen atoms needed: $8 - 0 = \mathbf{8\text{ hydrogen atoms}}$
* Oxygen atoms needed: $12\text{ (total needed)} - 10\text{ (from } \text{O}_2\text{)} = \mathbf{2\text{ oxygen atoms}}$

6. Determine the molecular formula:
Because one mole of the unknown compound must contain exactly 4 carbon atoms, 8 hydrogen atoms, and 2 oxygen atoms, its molecular formula is $\text{C}_4\text{H}_8\text{O}_2$. This matches Option D.

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WHY_OTHERS_WRONG:

• Option A is incorrect ($\text{C}_4\text{H}_8$): This formula lacks any oxygen atoms. If the reactant were $\text{C}_4\text{H}_8$, the left side of the equation would only contain 10 oxygen atoms (solely from the $5\ \text{O}_2$), failing to balance the 12 oxygen atoms required on the product side.
• Option B is incorrect ($\text{C}_4\text{H}_8\text{O}$): This formula contains only 1 oxygen atom. Combined with the 10 oxygen atoms from $5\ \text{O}_2$, the reactants would have a total of 11 oxygen atoms, which does not balance the 12 oxygen atoms in the products.
• Option C is incorrect ($\text{C}_4\text{H}_4\text{O}_2$): Although this formula contains the correct number of carbon and oxygen atoms, it provides only 4 hydrogen atoms. This is only half of the 8 hydrogen atoms required to form the 4 moles of water ($\text{H}_2\text{O}$) in the products.