Which of the following is the first step when solving a combustion thermochemistry problem involving an unknown hydrocarbon sample?

• A. Calculate the molar mass of the hydrocarbon.
• B. Determine the enthalpy change using Hess's Law.
• C. Write a balanced chemical equation for the combustion reaction.
• D. Find the empirical formula from the mass of oxygen consumed.

Answer: (C)

Verified step by step guidance

1. Identify the hydrocarbon and the products involved in the combustion reaction. Combustion of a hydrocarbon typically produces carbon dioxide (CO\_2) and water (H\_2O).
2. Write a general form of the combustion reaction for a hydrocarbon C\_xH\_y: \(\mathrm{C}_x\mathrm{H}_y + O_2 \rightarrow CO_2 + H_2O\).
3. Balance the chemical equation by ensuring the number of atoms of each element is the same on both sides. This involves adjusting coefficients for C\_xH\_y, O\_2, CO\_2, and H\_2O.
4. Once the balanced equation is established, use it to relate the amounts of reactants and products, which is essential for calculating enthalpy changes or empirical formulas.
5. After balancing, proceed to calculate quantities such as enthalpy change or empirical formula based on given data, using the balanced equation as the foundation.