The enthalpy of combustion of magnesium metal is difficult

Question

The enthalpy of combustion of magnesium metal is difficult to measure in a styrofoam cup calorimeter because the flame temperature can reach as high as 2200 K!

Mg(s) + 1⁄2O2(g) → MgO(s)

However, we can measure the enthalpies of the following reactions,

Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g) (1)

MgO(s) + 2HCl(aq) → MgCl2(aq) + H2O(l) (2)

Formation reaction of H2O(l) (3)

Which combination of the enthalpies of reactions 1, 2, and 3 would give the enthalpy of combustion 

of magnesium metal                                       

  A. (1) - (2) + 1⁄2 (3) B. (1)+(2)+(3) C. (1)-(2)+(3) D. (1) + (2) - 1⁄2 (3) E. (1)-(2)-(3) 

The reaction of an acid such as HCl with a base such as NaOH in water involves the exothermic reaction HCl(aq) + NaOH(aq) ---> NaCl(aq) + H2O In one experiment, a student placed 50.0 mL of 1.00 M HCl in a coffee-cup calorimeter and carefully measured its temperature to be 20.5°C. To this was added 50.0 mL of 1.00 M NaOH solution whose temperature was also 20.5°C. The mixture was quickly stirred, and the student noticed that the temperature of the mixture rose to 27.6°C. What was the heat of reaction? Ignore the heat capacity of the calorimeter in your calculation. The heat capacity of all solutions is 4.19 J/g K. The density of all solutions is 0.995 g/mL.

1. +59 kJ/mol.
2. +30 kJ/mol.
3. -118 kJ/mol.
4. -30 kJ/mol.
5. -59 kJ/mol.

Tammy applies the skills learned from the titration of vinegar exercise to the determination of the concentration of phosphoric acid necessary for her biology project. A total volume of 8.22 mL of 0.125 M potassium hydroxide solution was required to completely neutralize 14.32 mL of phosphoric acid (three acidic hydrogens). Calculate the concentration of her phosphoric acid solution. 0.0718 M

1. a 0.144 M
2. b 0.215 M
3. c 0.0239 M
4. d 0.645 M