<p>It would appear that the reaction is exothermic since the temperature increased.
Use Q = mc(delta T)
m = density(volume), so m = 100 g
delta T = 27.5 - 21 = 6.5
Q = 100(6.5)(4.18) = 2717 J,
but we're not done here, because this is for 50 mL of a 1.0 M solution
We need to find out how many moles of HCl were used.
So, 1 mol/1000mL x 50mL=.05mol HCl.</p>
<p>Now we have 2.717 kJ for every .05 mol HCl used, so divide #kJ by #mol and we get 54.34 kJ/mol, and since the reaction is exothermic, the answer is D.</p>