Which of the following is a list of the minimum amount of data needed for determining the molar enth — Thermodynamics Chemistry Question

### STEPS:

1. Identify the goal and formula for calorimetry:
The goal is to determine the standard molar enthalpy of solution ($\Delta H_{\text{soln}}$) of $\text{KCl}(s)$ in water, which represents the heat released or absorbed per mole of solute dissolved. The fundamental thermodynamic relationship used to measure heat transfer ($q$) in a calorimeter is:
$$q = m c \Delta T \quad$$
where $q$ is the heat of the solution, $m$ is the total mass of the solution, $c$ is the specific heat capacity, and $\Delta T$ is the change in temperature.

2. Determine the mass parameter ($m$):
The total mass of the aqueous solution is the sum of the components being mixed:
$$m_{\text{solution}} = \text{mass of KCl}(s) + \text{mass of }\text{H}_2\text{O}(l)$$
Therefore, to calculate $q$, the student must experimentally measure both the mass of $\text{KCl}(s)$ and the mass of $\text{H}_2\text{O}$.

3. Determine the temperature change parameter ($\Delta T$):
The change in temperature is defined as:
$$\Delta T = T_{\text{final}} - T_{\text{initial}}$$
Because the solute and solvent begin at the same temperature, the student must record the initial temperature of the water and the final temperature of the solution upon complete dissolution to find $\Delta T$.

4. Determine the moles of solute ($n$):
To convert the measured heat ($q$) to the *molar* enthalpy of solution ($\Delta H_{\text{soln}} = \frac{-q_{\text{solution}}}{n_{\text{KCl}}}$), the student needs to find the number of moles of solute ($n$) using:
$$n_{\text{KCl}} = \frac{\text{mass of KCl}}{\text{molar mass of KCl}} \quad$$
Since the molar mass of $\text{KCl}$ is a known constant derived from the periodic table, only the mass of $\text{KCl}(s)$ must be experimentally measured to find the moles.

5. Identify the minimum necessary data set:
Combining the requirements from steps 2, 3, and 4, the student must measure: the mass of $\text{KCl}(s)$, the mass of $\text{H}_2\text{O}$, the initial temperature of the water, and the final temperature of the solution. This list of parameters directly corresponds to Option C.

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### WHY_OTHERS_WRONG:

• Option A is incorrect: This option fails to list the mass of the water. Without knowing the mass of the solvent, the student cannot calculate the total mass ($m$) of the solution, preventing the calculation of heat ($q$).
• Option B is incorrect: This option fails to list the mass of $\text{KCl}(s)$. Without this measurement, the student can neither determine the total mass ($m$) of the solution to solve for $q$, nor calculate the moles of $\text{KCl}$ to determine the *molar* enthalpy change.
• Option D is incorrect: While this option contains all the correct variables, it unnecessarily includes atmospheric pressure. Dissolving a salt in a standard aqueous solution calorimeter occurs under constant atmospheric pressure; knowing the exact value of this pressure is not required to perform any of the calculations for $\Delta H_{\text{soln}}$.