What range of pH values could be achieved with a buffer consisting of nitrous acid (HNO2) and sodium nitrite (NO2−)?

It shows that the experiment is designed properly.

Ch4 and ph3 are both covalent since they are composed both of non-metals.    ch4 is non- polar because of  london forces and has a geometry of tetrahedral  ph3 , on the other hand, is umbrella shaped polar and has dipole-dipole intermolecular force (imf) . since ph3 is polar, it has a stronger imf