# chemistry

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Calculation of )Hf0 for Mg2+(aq.)

1. Assume the heat capacity of the final solution is 3.92 J K-1 g-1. Using the mass of the HCl
solution, calculate qcontents from equation (4.6). Calculate qsystem from equation (4.5).

2. Calculate the moles of Mg2+ produced, nrxn. Be sure to identify the limiting reagent in the
reaction.

3. Calculate the molar enthalpy change, )H0 rxn, for reaction (4.19). This is the same as )H0f for
Mg2+(aq.).Calculation of )Hf0 for MgO(s)

Calculation of delta Hf for MgO(s)

1. Again, assume the heat capacity of the final solution is 3.92 J K-1 g-1. Calculate qcontents and
qsystem.
2. Calculate the number of moles of Mg2+ (aq.) produced, nrxn.
3. Calculate the standard molar enthalpy change for reaction (4.20).
4. Average the values for )H0 rxn for part I and average the values for )H0
rxn for part II. Using these average values, calculate )H0f of MgO(s).

Questions
1. Why are both the mass and the volume of the hydrochloric acid solution recorded?

2. In this experiment it was assumed that the calorimeter neither absorbed nor released heat
(i.e., qcalorimeter = 0), but this is almost certainly not the case. Would the measured value of
)H0f (Mg2+(aq.)) be more or less exothermic if the calorimeter absorbed heat (that is, if
qcalorimeter > 0)? Explain why.

3. Explain why )H0rxn, for reaction (4.19) is the same as )H0f for Mg2+(aq.).

4. Use tabulated standard molar enthalpies of formation (from the textbook) to calculate )H0
rxn
for the following reactions:
1. 2NO(g) + O2(g) v 2NO2(g)
2. 2H2S(g) + 3O2(g) v 2SO2(g) + 2H2O(g)
3. Fe2O3(s) + 3CO(g) v Fe(s) + 3CO2(g)

5. Calcium sulphate reacts with carbon (in the form of graphite) according to the reaction:
CaSO4(s) + 1/2C(s, graphite) v CaO(s) + SO2(g) + 1/2CO2(g)

When 204.2 g of CaSO4(s) reacts with excess graphite at 298.15 K and 1 atm pressure, 456
kJ of heat are absorbed. Calculate the standard molar enthalpy of formation of CaSO4(s).
.

Data
Magnesium and hydrochloric
Run#1

mass of calorimeter+HCl/g =193.89g
mass of calorimeter/g=134.24g
mass og HCl/g=59.65g

mass of 50 mL beaker+Mg/g=30.7885g
mass of 50mL beaker/g=30.5655g
mass of Mg/g 0.2230g

initial temp22degree
final temp 30.1

run#2
mass of calorimeter+HCl/g =191.85g
mass of calorimeter/g=132.04g
mass og HCl/g=59.81g

mass of 50 mL beaker+Mg/g=31.7475g
mass of 50mL beaker/g=31.574g
mass of Mg/g 0.1727g

initial temp23degree
final temp 30.1
Part 2 Magnesium oxide+Hydrochloric
Run#1 Acid

mass of calorimeter+HCl/g =192.49g
mass of calorimeter/g=131.26g
mass og HCl/g=61.23g

mass of 50 mL beaker+Mg/g=32.2601g
mass of 50mL beaker/g=31.5502g
mass of Mg/g 1.2601g

initial temp23.5degree
final temp 28.0

run#2
mass of calorimeter+HCl/g =190.56g
mass of calorimeter/g=131.52g
mass og HCl/g=59.04g

mass of 50 mL beaker+Mg/g=32.2486g
mass of 50mL beaker/g=31.5532g
mass of Mg/g 0.6954g

initial temp24degree
final temp 28.0

• chemistry -

A pan containing 250 grams of water at 100oC is boiled dry. How many calories of heat are required to convert the liquid H2O to gaseous H2O?

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