When 150 g of ammonium nitrate dissolves in 450 g of water, the temperature of the solution decreases from 22 °C to 6.9 °C. What is the enthalpy of solution in joules per gram of ammonium nitrate?

Assume that the specific heat capacity of the solution is the same as that of pure water.

1 Answer
Ernest Z.
Jul 21, 2017

The enthalpy of solution is 250 J/g.

Explanation:

There are two heat transfers involved in this problem.

"heat of solution of NH"_4"NO"_3 + "heat lost by water" = 0
color(white)(mmmmmmm)q_1color(white)(mmmmml) +color(white)(mmmm) q_2color(white)(mmmm) = 0
color(white)(mmmmml)m_1Δ_text(sol)Hcolor(white)(mmmll) + color(white)(mmll)m_2CΔT color(white)(mm)= 0

Let's calculate the heats separately.

m_1 = "150 g"

q_1 = m_1Δ_text(sol)H = 150Δ_text(sol)Hcolor(white)(l) "g"

m_2 = "(150 + 450) g" = "600 g"
C = "4.18 J·g·°C"^"-1"
ΔT = "(6.9 - 22) °C" = "-15.1 °C"

q_2 = m_2CΔT = 600 color(red)(cancel(color(black)("g"))) × "4.184J"·color(red)(cancel(color(black)("g"^"-1"·"°C"^"-1"))) × ("-15.1") color(red)(cancel(color(black)("°C"))) = "-37 900 J"

q_1 + q_2 = 150Δ_text(sol)Hcolor(white)(l) "g" - "37 870 J"" = 0

Δ_text(sol)H = "37 900 J"/"150 g" = "250 J/g"

Note: The answer can have only two significant figures, because that is we have for the value of ΔT.

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