[gt5hz]

1. 100 mL of distilled water was measured in a graduated cylinder and poured into a clean, dry, plastic foam cup with its temperature was recorded to the nearest 0.1 degrees Celsius. 2 g of NaOH pellets was massed to the nearest 0.01 g on glazed paper and added to the water, stirred gently until the entire solid dissolved, while the temperature was checked constantly until the highest temperature was reached and recorded.

The question asks for the heat absorbed by the solution.

I know that Q = mcDELTAT. C will be specific heat capacity of water, delta T will be difference between initial and final temperature, but what is m? The mass of the NaOH pellets alone, or the mass of the pellets and the water together?

The question after that asks for the number of moles of NaOH used, which I have no problem with. However, the question after that asks for the the amount of heat evolved per mole of NaOH. What does that mean?

ALSO, how would I calculate the heat of reaction for this? NaOH (s) --> Na+ (aq) + OH- (aq)

[cliverlong]

<< snip >>
I know that Q = mcDELTAT. C will be specific heat capacity of water, delta T will be difference between initial and final temperature, but what is m? The mass of the NaOH pellets alone, or the mass of the pellets and the water together?

If you think that the heat is released after the water and NaOH are mixed, and you have a relatively small amount of NaOH, then the heat is distributed throughout the total amount of both compounds. You use the specific heat capacity of water as an approximation.

The question after that asks for the number of moles of NaOH used, which I have no problem with. However, the question after that asks for the the amount of heat evolved per mole of NaOH. What does that mean?

(heat or enthalpy) / moles

ALSO, how would I calculate the heat of reaction for this? NaOH (s) --> Na+ (aq) + OH- (aq)

I think you need to draw together the ideas of

• Enthalpy of solution
• Enthalpy of hydration (note starting point is gaseous ions)
• Born Haber cycle to calculate the enthalpy of reaction for ionic compounds

The following may help

http://www.avogadro.co.uk/definitions/hsolution.htm
http://www.avogadro.co.uk/definitions/hhydr.htm
http://www.science.uwaterloo.ca/~cchieh/cact/applychem/hydration.html

Clive