Which of the following chemical equations is not balanced? A) C5H12 + 8O2 → 5CO2 + 6H2O B) H3PO4 + 3NaOH → Na3PO4 + 3H2O C) PCl5 + 3H2O → H3PO4 + 5HCl

To determine which equation is not balanced, we count the atoms of each element on both sides of the reaction arrow for each equation.

A) $\text{C}_5\text{H}_{12}(\text{l}) + 8\text{O}_2(\text{g}) \rightarrow 5\text{CO}_2(\text{g}) + 6\text{H}_2\text{O}(\text{g})$

• Carbon (C): Reactants = 5, Products = $5 \times 1 = 5$. (Balanced)
• Hydrogen (H): Reactants = 12, Products = $6 \times 2 = 12$. (Balanced)
• Oxygen (O): Reactants = $8 \times 2 = 16$, Products = $(5 \times 2) + (6 \times 1) = 10 + 6 = 16$. (Balanced) Equation A is balanced.

B) $\text{H}_3\text{PO}_4(\text{aq}) + 3\text{NaOH}(\text{aq}) \rightarrow \text{Na}_3\text{PO}_4(\text{aq}) + 3\text{H}_2\text{O}(\text{l})$

• Hydrogen (H): Reactants = $3 + (3 \times 1) = 6$, Products = $3 \times 2 = 6$. (Balanced)
• Phosphorus (P): Reactants = 1, Products = 1. (Balanced)
• Oxygen (O): Reactants = $4 + (3 \times 1) = 7$, Products = $4 + (3 \times 1) = 7$. (Balanced)
• Sodium (Na): Reactants = $3 \times 1 = 3$, Products = 3. (Balanced) Equation B is balanced.

C) $\text{PCl}_5(\text{g}) + 3\text{H}_2\text{O}(\text{g}) \rightarrow \text{H}_3\text{PO}_4(\text{l}) + 5\text{HCl}(\text{aq})$

• Phosphorus (P): Reactants = 1, Products = 1. (Balanced)
• Chlorine (Cl): Reactants = 5, Products = $5 \times 1 = 5$. (Balanced)
• Hydrogen (H): Reactants = $3 \times 2 = 6$, Products = $3 + 5 = 8$. (Not balanced)
• Oxygen (O): Reactants = $3 \times 1 = 3$, Products = 4. (Not balanced) Equation C is not balanced.

D) $\text{Pb(NO}_3)_2(\text{aq}) + 2\text{HCl}(\text{aq}) \rightarrow \text{PbCl}_2(\text{s}) + 2\text{HNO}_3(\text{aq})$

• Lead (Pb): Reactants = 1, Products = 1. (Balanced)
• Nitrogen (N): Reactants = $1 \times 2 = 2$, Products = $2 \times 1 = 2$. (Balanced)
• Oxygen (O): Reactants = $2 \times 3 = 6$, Products = $2 \times 3 = 6$. (Balanced)
• Hydrogen (H): Reactants = $2 \times 1 = 2$, Products = $2 \times 1 = 2$. (Balanced)
• Chlorine (Cl): Reactants = $2 \times 1 = 2$, Products = 2. (Balanced) Equation D is balanced.

The equation that is not balanced is C.

The final answer is $\boxed{\text{C}}$.