226
Thermochemistry
24. A metal Y, whose specific heat is 0.0312 cal/g °C, forms two chlorides whose
compositions are 35.10% Cl and 15.25% Cl.
(a) What is the exact atomic weight of the metal?
(b) What are the formulas of the chlorides?
25. A Dewar flask (vacuum-jacketed bottle) is used as a calorimeter, and the
following data are obtained. Measurements in parts (a) and (b) are made to
obtain the heat capacity of the calorimeter, and parts (c) and (d) are performed
on an unknown metal.
(a) Calorimeter with 200 g of water has a temperature of 23.7°C.
(b) When 50.0 g of water at 99.1°C are added to the calorimeter and water in
(a), the resultant temperature is 36.0°C.
(c) Calorimeter with 200 g of water has a temperature of 20.6°C.
(d) A 91.5 g sample of metal at 99. PC added to the calorimeter and water in
(c) gives a resultant temperature of 21.5°C.
Calculate the approximate atomic weight of the metal.
26. A 200 g sample of an unknown high-boiling liquid is put into the calorimeter
calibrated in Problem 25, and its temperature is observed to be 24.2°C. Then
55.3 g of copper at 180.0°C are added to the liquid in the calorimeter to give a
resultant temperature of 28.0°C. Calculate the specific heat of the liquid. Use
the approximate specific heat of copper.
27. Calculate the heat capacity of a calorimeter (Figure 14-1) that, when containing 250 ml of water, requires a current of 0.650 amp passing for 5 min 25 sec
through the 8.35 ohm immersed resistance in order to raise the water temperature from 25.357°C to 26.213°C.
28. Compute the following quantities, using the calorimeter calibrated in Problem
27.
(a) The specific heat of an unknown metal. First, 40.0 g of a finely divided
sample are stirred with 220 ml of water; then a current of 0.720 amp for 12
min 24 sec raises the temperature from 25.265°C to 27.880°C.
(b) The specific heat of an unknown liquid. First, 250 g of it are placed in the
calorimeter in place of the water; then a current of 0.546 amp is passed for
4 min 45 sec, raising the temperature from 26.405°C to 27.033°C.
(c) The enthalpy change per mole of precipitated BaSO 4 when 125 ml of 0.500
M BaCl 2 solution are mixed in the calorimeter with 125 ml of 0.500 M
Na 2 SO 4 solution. A current of 0.800 amp passing for 5 min 41 sec through
the heater immersed in the reaction mixture caused a temperature rise
1.500 times larger than that observed for the reaction. Also write the
equation for the reaction involved.
(d) The enthalpy change per mole of NH 4 NO 3 when 16.0 g of solid NH 4 NO 3
are dissolved in 250 ml of water in the calorimeter. This is the so-called
"heat of solution." After the salt is dissolved, a current of 1.555 amp
passing for 4 min 16 sec is required in order to regain the initial temperature of the water. Write the equation for this "reaction."
(e) The enthalpy of fusion per mole of diiodobenzene, C 6 H 4 I 2 . When 20.0 g of
diiodobenzene are added to 250 ml of water in the calorimeter, the crystals
(being immiscible with water and also more dense) sink to the bottom. As
Thermochemistry
24. A metal Y, whose specific heat is 0.0312 cal/g °C, forms two chlorides whose
compositions are 35.10% Cl and 15.25% Cl.
(a) What is the exact atomic weight of the metal?
(b) What are the formulas of the chlorides?
25. A Dewar flask (vacuum-jacketed bottle) is used as a calorimeter, and the
following data are obtained. Measurements in parts (a) and (b) are made to
obtain the heat capacity of the calorimeter, and parts (c) and (d) are performed
on an unknown metal.
(a) Calorimeter with 200 g of water has a temperature of 23.7°C.
(b) When 50.0 g of water at 99.1°C are added to the calorimeter and water in
(a), the resultant temperature is 36.0°C.
(c) Calorimeter with 200 g of water has a temperature of 20.6°C.
(d) A 91.5 g sample of metal at 99. PC added to the calorimeter and water in
(c) gives a resultant temperature of 21.5°C.
Calculate the approximate atomic weight of the metal.
26. A 200 g sample of an unknown high-boiling liquid is put into the calorimeter
calibrated in Problem 25, and its temperature is observed to be 24.2°C. Then
55.3 g of copper at 180.0°C are added to the liquid in the calorimeter to give a
resultant temperature of 28.0°C. Calculate the specific heat of the liquid. Use
the approximate specific heat of copper.
27. Calculate the heat capacity of a calorimeter (Figure 14-1) that, when containing 250 ml of water, requires a current of 0.650 amp passing for 5 min 25 sec
through the 8.35 ohm immersed resistance in order to raise the water temperature from 25.357°C to 26.213°C.
28. Compute the following quantities, using the calorimeter calibrated in Problem
27.
(a) The specific heat of an unknown metal. First, 40.0 g of a finely divided
sample are stirred with 220 ml of water; then a current of 0.720 amp for 12
min 24 sec raises the temperature from 25.265°C to 27.880°C.
(b) The specific heat of an unknown liquid. First, 250 g of it are placed in the
calorimeter in place of the water; then a current of 0.546 amp is passed for
4 min 45 sec, raising the temperature from 26.405°C to 27.033°C.
(c) The enthalpy change per mole of precipitated BaSO 4 when 125 ml of 0.500
M BaCl 2 solution are mixed in the calorimeter with 125 ml of 0.500 M
Na 2 SO 4 solution. A current of 0.800 amp passing for 5 min 41 sec through
the heater immersed in the reaction mixture caused a temperature rise
1.500 times larger than that observed for the reaction. Also write the
equation for the reaction involved.
(d) The enthalpy change per mole of NH 4 NO 3 when 16.0 g of solid NH 4 NO 3
are dissolved in 250 ml of water in the calorimeter. This is the so-called
"heat of solution." After the salt is dissolved, a current of 1.555 amp
passing for 4 min 16 sec is required in order to regain the initial temperature of the water. Write the equation for this "reaction."
(e) The enthalpy of fusion per mole of diiodobenzene, C 6 H 4 I 2 . When 20.0 g of
diiodobenzene are added to 250 ml of water in the calorimeter, the crystals
(being immiscible with water and also more dense) sink to the bottom. As