Consider the following reaction:

2 Mg(s) + O2(g) > 2 MgO(s) ∆H= -1204 kJ

Calculate the amount of heat transferred when 2.500 g of Mg(s) reacts at constant pressure.

a. -1204 kJ
b. -120.4 kJ
c. -60.20 kJ
d. +60.20 kJ
e. +120.4 kJ

c. -60.20 kJ

Explanation:

Given data:

Heat produced by 2 moles of Mg = -1204 kj

Heat produced by 2.500 g of Mg = ?

Solution:

Chemical equation:

2Mg + O₂     →    2MgO         ∆H = -1204 Kj

No we will calculate the number of moles of Mg:

Number of moles = mass / molar mass

Number of moles = 2.500 g/ 24 g/mol

Number of moles = 0.1 mol

Heat produced by one mole of Mg = -1204 kj/2 = -602 kj

Heat produced by 0.1 mole of Mg = 0.1 × -602 kj = -60.2 kj

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