To calculate the heat of mixing, we need to first determine the number of moles of HCl and NaOH reacting in the solution.Moles of HCl = volume in L concentration in mol/L Moles of HCl = 25 mL 1 L/1000 mL 1.0 mol/L = 0.025 molMoles of NaOH = volume in L concentration in mol/L Moles of NaOH = 25 mL 1 L/1000 mL 1.0 mol/L = 0.025 molSince the moles of HCl and NaOH are equal, the reaction will go to completion and all of the HCl and NaOH will react.Now, we can use the heat of neutralization to calculate the heat of mixing.Heat of mixing = moles of HCl heat of neutralization Heat of mixing = 0.025 mol -57.1 kJ/mol = -1.4275 kJThe heat of mixing when 25 mL of 1.0 M HCl solution is added to 25 mL of 1.0 M NaOH solution at 25C is -1.4275 kJ.