NY Regents Chemistry Exam June 2022
38 What is the amount of heat required to melt 43 grams of solid magnesium oxide at its melting point? The heat of fusion is 1.9 * 10^3 J/g. (1) 2.3*10^2 J
(2) 4.4*10^1 J
(3) 8.2*10^4 J
(4) 3.4*10^5 J
Explanation: Table T on the Reference Table gives three formulas for calculating heat. q=mCΔT q = mHf q = mHv. Which one do we use? This question asks about heat needed for melting. For melting, we use q=mHf where m is mass and Hf is the heat of fusion. Heat of fusion is given in the question and so is the mass. All we need to do is multiply them together.
Ps: q = mHv is used for boiling and q=mCΔT is used where there is a change of temperature without the phase change.