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?
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
