Question

A bomb calorimeter has a heat capacity of 1769 J/oC. A 0.1375 –g piece of solid magnesium is burned in the calorimeter that contains exactly 300 g of water. The observed temperature increase is 1.126oC. Calculate the heat given off by the burning magnesium in kJ/g and kJ/mol. Show all work and correct significant figures.

Answer 1

Given,

Mass of water = 300 g

Delta T = 1.126 degree C

Mass of Mg = 0.1375 g

Heat capacity of calorimeter = 1769 J / degree C

We know that,

Molar Mass of Mg = 24.3 g / mol

Cp for water = 4.18 J / g degree C

=> Moles of Mg = 0.1375 / 24.3 = 0.00566 moles

Total heat given off by Mg = Total heat absorbed by water and calorimeter

Total heat absorbed by water = m Cp delta T = 300 x 4.18 x 1.126 = 1414 J

Heat absorbed by calorimeter = 1769 x 1.126 = 1991.9 J

=> otal heat absorbed by water and calorimeter = 1414 + 1991.9 = 3405.9 J

=> 0.1375 g of Mg releases 3405.9 J

=> 1 g releases 3405.9 / 0.1375 = 24770.2 J

24.77 kJ / g

0.00566 moles of Mg releases 3405.9 J

=> 1 mole releases 3405.9 / 0.00566 = 602385.2 J

602.39 kJ / mol