Okay i hate thermochem problems and never get them right, please help. I will on

Question

Okay i hate thermochem problems and never get them right, please help. I will only award 1500 points to the person who does it in understandable steps and units with significant figures applied.

The following experiment was performed. The mass of an empty coffee cup calorimeter was 5.25 g. To the calorimeter was added 80.0 mL of 1.50 M hydrobromic acid; the mass of the calorimeter and solution was 85.15 g. The temperature of the solution in the calorimeter was 20.00 degree C. To the calorimeter w as added 45.0 mL of a 2.00 M aqueous solution of sodium hydroxide, also at 20.00 degree C. After the two solutions had been mixed and the temperature of the resultant solution was no longer changing, the temperature of the solution in the calorimeter was 28.57 degree C, and the mass of the calorimeter and contents was 132.75 g. The specific heat of liquid water is 4.18 J/g degree C; the specific heat of the solution present in the calorimeter at the end of the experiment is 4.20 J/g- degree C. The energy change per mole of hydrobromic acid that reacts in the reaction between hydrobromic acid and sodium hydroxide is -52.4 kJ/mol. Calculate the heat capacity of the calorimeter.

Explanation / Answer

Step 1: Determine heat released:


The chemical reaction involved is: HBr(aq) + NaOH(aq) => NaBr(aq) + H2O(l)

Theoretical moles of HBr : NaOH = 1 : 1


Actual moles of HBr used = volume x concentration of HBr

= 80.0/1000 x 1.50 = 0.12 mol


Actual moles of NaOH used = volume x concentration of NaOH

= 45.0/1000 x 2.00 = 0.09 mol


Actual moles of HBr : NaOH = 0.12 : 0.09 = 1.33 : 1

Since HBr in in excess => NaOH is the limiting reagent


Moles of H2O formed = moles of NaOH = 0.09 mol

Heat released by reaction = moles of water formed x (-enthalpy of neutralization)

= 0.09 x 52.4 = 4.716 kJ = 4716 J


Step 2: Determine heat absorbed:


Mass of solution = final mass of solution and calorimeter - mass of calorimeter

= 132.75 - 5.25 = 127.5 g


Heat absorbed by solution = mass x specific heat x temperature change of solution

= 127.5 x 4.20 x (28.57 - 20.00) = 4589.235 J


Let C be the heat capacity of the calorimeter

Heat absorbed by calorimeter = heat capacity x temperature change of calorimeter

= C x (28.57 - 20.00) = 8.57C J


Step 3: Determine heat capacity:


Total heat absorbed = heat released by reaction

4589.235 + 8.57C = 4716

8.57C = 126.765

C = 126.765/8.57 = 14.8 J/C


Heat capacity of calorimeter = C = 14.8 J/C (3 significant figures)

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