Question

Compound	Δ Hrxn (kJ/mol)
NH4NO3	+ 25.7
KCl	+ 17.2
NaOH	-44.5
KOH	-57.6

You are given 6.9 grams of an unknown salt. You dissolve it in 56.5 mL of water.
As the salt dissolves, the temperature of the solution changes from an initial temperature of 28.9°C to a final temperature of 22.9°C.
Based on the chart of ΔH values and the temperature change, you can determine which salt you have.

What is the temperature change?
Δ T =  ^o C

q_surr = C x mass x ΔTThis is the ΔT component of the expression:

where q_surr is the heat lost or gained by the contents of the calorimeter due to the chemical reaction.
What is the mass that is changing temperature in this experiment?
mass =  g

Assume that the heat capacity, C, of the solution is the same as that of water, C = 4.184 J/g ^oC.
Solve for q_surr.
q_surr =  J

What is the value of q_sys?

The change in enthalpy, ΔH, of this reaction is equal to q_sys, since it was carried out under conditions of constant external pressure.
Based on the sign of ΔH (or q_sys) the dissolution of the unknown salt is  endothermic exothermic  .

When 6.9 g of salt are dissolved in 56.5 mL of water, 1591.6 J of heat are absorbed by the system.
How much heat would be absorbed if only 1.00 g of salt had been dissolved in the water?
q_sys =  J/g

Finally, multiply the kJ/g by the molar masses (g/mol) of possible salts, to get kJ/mol.In the table, the ΔH values are given in kJ/mol.
Convert your last answer to kJ to give kJ/g.

The reaction of your unknown salt with water was endothermic. So, you will only need to look at salts with positive ΔH values.
formula of unknown =

Answer 1