Practice Problems p. 525
12. A 90g sample of an unknown metal absorbed 25.6J of heat as its
temperature increased 1.18oC. What is the specific heat of the metal?
J/goC = 25.6J
X
1
X
1
= 0.24 J/goC
1
90g
1.18oC
13. The temperature of a sample of water increases from 20.0oC to 46.6oC
as it absorbs 5650J of heat. What is the mass of the sample? specific heat
= 4.184J/goC
T 46.6 - 20 = 26.6oC
g = goC
X
5650J
X
1 = 50.8g
4.184J
1
26.6oC
p.532 practice problems
23. Calculate the heat requried to melt 25.7g of solid methanol, CH3OH, at
its melting point.
Hfus = 3.22kJ/mol
25.7g X 1 mol
X
3.22kJ = 2.58kJ
1
32.04g
1 mol
24. How much heat evolves when 275g of ammonia gas, NH3, condenses to a
liquid at its boiling point?
Hvap = 23.3kJ/mol
275g X
1 mol
X 23.3kJ = 376kJ
1
17.03g
1 mol
p. 537
32. Use equations a and b to determine
H for the following reaction:
2CO + 2NO
2CO2 + N2
H = ?
a. 2CO + O2
2CO2
H = -566 kJ
b. N2 + O2
2NO
H = -180.6 kJ
(reverse the second equation and add the equations together)
2NO
N 2 + O2
+180.6kJ
+ 2CO + O2
2CO2
-566 kJ
2CO + O2 + 2NO
2CO2 + N2 + O2
-385.4 kJ
2CO + 2NO
2CO2 + N2
35. Use standard enthalpies of formation to calculate
reaction:
Hrxn for the following
4NH3(g) + 7O2(g)
4NO2(g) + 6H2O(l)
Hf
kJ/mol
NH3(g) -45.9
NO2(g) 33.2
H2O(l) -285.8
((4)(33.2) + (6)(-285.8)) - ((4)(-45.9) + (7)(0)
= -1580.8 - -183.6 = -1397.2 kJ