A 1.67-g sample of solid silver reacted in excess chlorine gas to give a 2.21-g sample of pure solid AgCl.The heat given off in this reaction was 1.96 kJ at constant pressure.Given this information,what is the enthalpy of formation of AgCl(s)?

Question

Quizplus · Accepted Answer

First, we need to find the amount of moles of Ag that reacted:
moles of Ag = 1.67 g / 107.87 g/mol = 0.0155 mol
Since the reaction produced pure AgCl, the same amount of moles of Ag are present in AgCl:
moles of AgCl = 0.0155 mol
Using the mass of AgCl produced, we can find the molar mass of AgCl:
molar mass of AgCl = 2.21 g / 0.0155 mol = 142.58 g/mol
Now, we can use the heat given off in the reaction and the amount of moles of AgCl produced to find the enthalpy of formation of AgCl:
ΔH°f = -q/n
where q is the heat given off in the reaction (-1960 J converted to kJ = -1.96 kJ), and n is the number of moles of AgCl produced (0.0155 mol)
ΔH°f = -(-1.96 kJ / 0.0155 mol) = 126.45 kJ/mol
Rounding to three significant figures, the answer is 127 kJ/mol, which is choice A.

A 167-G Sample of Solid Silver Reacted in Excess Chlorine Gas