Question

NaCl has a `DeltaH_(fus)` = 30.2 kJ/mol. What is the mass of a sample of NaCl that needs 732.6 kJ of heat to melt completely?

Answer 1

`"559 g Na"` would need `"732.6 kJ"` of heat to melt completely.

Explanation:

Determine moles Na by dividing by its heat of fusion.

`732.6color(red)cancel(color(black)("kJ"))xx(1"mol Na")/(30.2color(red)cancel(color(black)("kJ")))="24.3 mol Na"`

Determine mass of Na by dividing by its molar mass (atomic weight in g/mol).

`24.3color(red)cancel(color(black)("mol Na"))xx(22.98977"g Na")/(1color(red)cancel(color(black)("mol Na")))="559 Na"` rounded to three sig figs