[tonigerco]

A student wants to measure the heat of
solution of sodium hydroxide. To do this,
he places 100 g of water in a beaker. The
temperature of the water is 15°C. He
weighs out 10.0 g of solid sodium
hydroxide. He pours all of this into the
water in the cup, stirs to help the solid to
dissolve, and measures the maximum
temperature reached by the mixture as
38°C.

(a) Use the equation ΔH = –mCΔT to calculate the amount heat given off by dissolving
the sodium hydroxide in 100 g of water.
The specific heat of water is 4.18 J g–1 °C–1.
Show the relevant formula and working

(b) Use your answer from (a) to show that the heat of solution of sodium hydroxide is –38
kJ mol–1. The molar mass of sodium hydroxide is 40.0 g.
Show the relevant formula and working



So... for a). ΔH = -m×C×ΔT
Where ΔH is the amount of heat absorbed or given
m is the mass of the water - 100g
C is the heat of the water - 4.18J
ΔT is the change in temperature - 23°C

ΔH = -100×4.18×23 = -9614J so -9.6kJ (negative because the solution is exothermic heat)

B) (not sure what formula i should be using???
But since 10g of NaOH is -9.6. 40g of sodium would be 9.6 x 4= -38.4. Is this correct?

Thank you. Toni.

[mikasaur]

That looks like you got to the right answer, but your work takes a lot of shortcuts and is at risk of getting you in trouble for harder questions.

It's kind of tough to write it out on the forum (you could use our built in LaTeX interpreter) but in your actual homework assignment you'd probably want to be a bit more rigorous in figuring out (b). You'd basically be doing conversion from

J / g NaCl
to
kJ / mol NaCl