Question

Define $P^{H}$ and $P^{OH}$. Write the relation between $P^{H}$ and $P^{OH}$ which indicate different degree of aqueous solution

Answer 1

Texinfo is yet another macro package which defines its own markup language, which may be processed by TeX to produce dvi (texi2dvi) or pdf (texi2pdf). You may include arbitrary (roughly, plain) TeX code inside the @iftexand @end iftex commands; but then, anything within this conditional will be ignored in conversion to other formats (html, info, etc.). texi2[dvi|pdf] are shell scripts that automate the processing of Texinfo files to produce documentation for printing, analogous to makeinfo, a program which processes the same documentation to produce online documentation in info or html formats, for instance.
texi2pdf may also process automatically LaTeX files, by checking the filename extension. Thus, texi2pdf filename.texautomatically processes filename.tex as a LaTeX file, with its main byproducts (bibtex and makeindex).
You may trick texi2pdf into processing Plain TeX files with the strategies suggested by Fran; but then you would be only forcing the texi2pdf script to use the pdftexengine with the pdftex format on the Plain TeX file, and the result would be the same as using the command pdftex filename.tex