Which are valid ways to make an ammonia/ammonium buffer for use in the laboratory?

See the explanation below, please.

Explanation:

A buffer solution (also called buffer / regulatory solution) corresponds to a mixture of high concentrations of an acid with its conjugate base, which allows maintenance of the pH when amounts of strong acids or strong bases are added. A buffer solution can be NH3 / ClNH4 (ammonia which is the base, and ammonium chloride, the acid).

  For example, to prepare said buffer, X volume of deionized water is placed, the ammonium chloride to be used is measured on an analytical scale, then dissolved in water, using a stirrer. On the other hand, ammonium hydroxide is added in volume X of deionized water. Both solutions are combined, increasing the flask. The pH is measured using a pH meter.

when [oh-] is 10-5  m, and [h3o+] is 10-9  m, the poh is 5 and ph is 9. again in both cases the sum of ph and poh is 14. however, the hydrogen ion concentration is not always going to be equal to exactly 1 x 10 raised to a negative number. for example, we skipped over the value of 2.0 x 10-7.