# What is the standard heat of formation of the nitrate ion, based on the standard enthalpy of formation of silver nitrate?

Given the standard enthalpy of formation of $\ce{AgNO3(aq)} = -99 \text{ kJ/mol}$ and the standard enthalpy of formation of $\ce{Ag+(aq)} = +105 \text{ kJ/mol}$, what's the standard heat of formation of $\ce{NO3^{-}(aq)}$?

How can I know this if the standard enthalpy of reaction is not known? This is self test 10.1 in Atkins, PC.

• We've edited your question to make the chemistry look prettier. You can check out the faq for more information on how to do this yourself. I also added the homework tag. Even though this is not a homework question, you will want a homework style answer - one in which you are shown how to go about solving the problem, but you are not given the answer. – Ben Norris Apr 24 '13 at 11:01

Silver nitrate, $$\ce{AgNO3}$$ is a strong electrolyte. Based on the information given, I expect you are meant to assume that it dissociates completely when dissolved in water:
$$\ce{AgNO3(aq)-> Ag+(aq) + NO3- (aq)}$$
Thus, we can make the assumption that $$\ce{AgNO3(aq)} \equiv \ce{Ag+(aq) + NO3- (aq)}$$
$$\Delta_\mathrm{f} H (\ce{AgNO3(aq)}) = \Delta_\mathrm{f} H (\ce{Ag+(aq)})+\Delta_\mathrm{f} H (\ce{NO3- (aq)})$$