Chemistry question( finding the mass of ethanol in my problem)

Question

Find the mass of ethanol that requires 67000J of heat to melt solid ethanol (Hf=109 J/g) to liquid at 159K.

This question is really confusing for me and I want to understand how to do it. If someone can explain it step by step, I would really appreciate it.

Answer 1

The melting point of ethanol is 159K and so at this temperature it can exist as both a solid and a liquid. This is just like water at 0ºC as it can exist as ice (solid) and liquid. What we're looking at there is nothing more than a phase change, i.e. going from solid to liquid. All of the heat put in is used to melt the ethanol and there is no change in temperature. So, the equation is heat = mass x ∆Hfusion

q = m∆Hf

67000 J = m (109 J/g)

m = 67000 J/109 J/g

m = 615 g