A student tests the acidity of nitric acid (HNO3) by dissolving a sample of HNO3 in a solution of liquid methane. He then uses an electronic tool to measure the solution and determines that it is highly acidic. What is wrong with the student’s experiment?

Question

aaronq · Answer

the instrument he used was a pH meter, these measure the activity (roughly concentration) of the $H_3O^+$ (hydronium) ion concentration in a solution. But in order to do this the acid needs a medium that it can dissociate in, water is the most commonly used solvent - the ability for an acid to ionize depends on how it interacts with the solvent and the energy profile that comes from it, in short it all comes down to polarity of molecules. Polar molecules associate favourably with other polar molecules. SO HNO3 would ionize in water like this:

$HNO_3 +H_2O\rightleftharpoons H_3O^++NO_3^-$

In constrast, methane in a non-polar molecule, so the ionization of $HNO_3$ is energetically unfavourable and will not happen. The pH meter will not be able to measure the activity of the $H_3O^+$ because there are none.