A student determines the molar enthalpy of solution, \(\Delta H_{\text{soln}}^\circ\), for \(\text{KNO}_3\text{(s)}\) in water using a coffee-cup calorimeter. The dissolution process is represented by the equation below.

\[ \text{KNO}_3\text{(s)} \rightarrow \text{K}^+\text{(aq)} + \text{NO}_3^-\text{(aq)} \quad \Delta H_{\text{soln}}^\circ > 0 \]

The student adds a weighed sample of \(\text{KNO}_3\text{(s)}\) to a known mass of distilled water at \(22.0\ ^\circ\text{C}\), stirs the mixture, and records the minimum temperature reached. In calculating \(\Delta H_{\text{soln}}^\circ\), the student assumes that the entire mass of \(\text{KNO}_3\text{(s)}\) dissolved and that no heat was exchanged with the surroundings. However, a small amount of solid \(\text{KNO}_3\) remained undissolved at the bottom of the cup at the end of the trial. Which of the following correctly predicts the effect of the undissolved solid on the calculated value of \(\Delta H_{\text{soln}}^\circ\), and provides the best justification?