Question

If you wanted to decrease the temperature of 100 g of water from 25 degrees C to 5 degrees C, how much NH_4NO_3 would you have to add?

Answer 1

ANSWER:

      The heat is absorbed by NH4NO3 when dissolved in water and temperature goes down. The change in heat is to be determined in order to find out the quantities of the NH4NO3.

      The following equation can be used to find out the total heat transfer in the process.

Q	=	m	c	T
heat absorbed/ lost (joules)	=	mass of water (g)	specific heat of water (J/gC)	temperature change (C)

      The specific heat of water = 4.18 J/gC

      Remember the amount of heat absorbed (or lost) by H2O is equal to the amount of heat lost (or absorbed) by the NH4NO3.

For ammonium nitrate solution, the heat absorbed,

Q = mcT by replacing the values from given problems,

Q = (100g)(4.18J/gC)(20C)

Q = 8.36 KJ

      Molar heat of solution NH4NO3 = total heat solution / moles solute used= + 25.7KJ/mol        

So, amount of ammonium nitrate = 8.36/25.7 =0.325 moles

Therefore the quantities of ammonium nitrate required= 80.05x0.325 = 26.016 g