Question

A 0.1326 g sample of magnesium was burned in an oxygen bomb calorimeter. the total heat capacity of the calorimeter plus water was 5,760 j/°c. if the temperature rise of the calorimeter with water was 0.570°c, calculate the enthalpy of combustion of magnesium. mg(s) + 1/2o2(g) →mgo(s)

Answer 1

The enthalpy of combustion of magnesium is -601.3 kj/mol

calculation

ΔH = cΔT

C( heat capacity) =5,760 j/°c

ΔT = 0.570°c

ΔH = 5760 j/°c x 0.570 °c = 3283.2 j

convert 3283.2 j into Kj

1 kj = 1000 j

kj ? =3283.2 j

by cross multiplication

={ (3283.2 j x 1 kj) / 1000 j} =3.2832 kj

Find the moles of Mg

moles = mass÷ molar mass

from periodic table the molar mass of Mg = 24.3 g/mol

= 0.1326 g÷ 24.3 g/mol =0.00546 moles

find ΔH in KJ/mol

= 3.2832 kj/ 0.00546 moles = 601.3 Kj/mol

since heat is released during combustion the ΔH is = - 601.3 kj/mol