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