kaplan subject test chem question

[Strikeout13GB]

How many calories are needed to convert 8 grams of ice at –23 °C to water at 52 °C? (The specific heat of water and ice is 1 cal/(g)(°C), and the heat of fusion of water is 80 cal/g.)
A. (75)(8) + (80)(10)
B. (75)(10) + (80)(8)
C. (75)(10) + (80)(10)
D. (75)(8) + (80)(8)
E. (23)(8) + (52)(8)

I thought when you used q=n(delta)Hvap or H fus you used the moles, this says your still yous m (mass) which is 8

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[cm8a12321]

How many calories are needed to convert 8 grams of ice at –23 °C to water at 52 °C? (The specific heat of water and ice is 1 cal/(g)(°C), and the heat of fusion of water is 80 cal/g.)
A. (75)(8) + (80)(10)
B. (75)(10) + (80)(8)
C. (75)(10) + (80)(10)
D. (75)(8) + (80)(8)
E. (23)(8) + (52)(8)

I thought when you used q=n(delta)Hvap or H fus you used the moles, this says your still yous m (mass) which is 8

This problem is a phase diagram related to temperature and heat.

g--> //
(l-g) ----
l--> //
(s-l) ----
s--> //

What happens is that you have phase and temperature change from solid to liquid when heat is added. In order to find heat, you would not only need to find the q (heat) of fusion (energy to get the solid to become liquid)=m(delta H), but also the heat that reflect the temperature change=mc(delta T).

This is how it should look like:

mc(delta T)+m(delta Hfusion)=total heat
8(1)(52-(-23))+ 8(80)=8(75)+8(80) cal-->which is D.

 

[Strikeout13GB]

yea i get that but i thought when you use q=Hvap or Hfus you used n as in moles not mass

 

[UCB05]

yea i get that but i thought when you use q=Hvap or Hfus you used n as in moles not mass

"(The specific heat of water and ice is 1 cal/(g)(°C), and the heat of fusion of water is 80 cal/g.)"

You're given the specific heat and heat of fusion of water in gram units. Use the gram units

 

[cm8a12321]

yea i get that but i thought when you use q=Hvap or Hfus you used n as in moles not mass

You have to look at the units of the Hvap or Hfus and specific heat constant to see if it is moles or grams, just like what UCB05 wrote above.

 

[chromosome21]

yes, it can be possible.

q = n x delta H ==> here, n = moles, and units of (delta)H = jouls/mol...
here, units of q = jouls


q = m x delta H ==> here, m = grams, and units of (delta)H = cal/g..
here, units of q = cal

1 cal = ~4.2 jouls

 

[Strikeout13GB]

ooooooooooo haha i see now, wow idk how i've got this far without realizing that thank you all very much