Question

149.3 g of H2O at 95 ◦C is poured over 412 g Fe at 5 ◦C in an insulated vessel. What is the final temperature? The specific heat of H2O is 4.184 J /g· ◦C, the specific heat of Fe is 0.444 J/g · ◦C. Answer in units of ◦C.

I believe I am supposed to use the equation -Q=Q, where each Q=mc(change in T). But how do I know whether the water or the iron is the negative Q?

Answer 1

You are correct, but you needn't worry about the signs so much. Just remember that the negative sign is used to denote a loss of energy; since the water is hotter, it will be losing energy (-Q) and the iron will gain energy (Q). Now, we substitute the values:
-149.3 * 4.184 * (T - 95) = 412 * 0.44 * (T - 5)
Solving this equation for T,
T = 74.8 °C