Answer
$T_f = 390 K$ or $117.4 ^o C$
Work Step by Step
$\Delta U = 1500 J - 2100 J $
$\Delta U = -600 J $
To find the temperature, we use the total internal energy formula and solve for $T_f$
$\Delta U = n (\frac{3}{2} R)( T_f - T_i)$
$T_f = \frac{2\Delta U}{3nR} + T_i$
$T_f = \frac{2(-600 J )}{3(5)(8.314 J/mol.K)} + (127^o C + 273 K)$
$T_f = 390.4 K$ or $117.4 ^o C$
