$5.$ A water analyst uses NH$4$K$2$PO$4$ to prepare standards for NH$4+,$ K and P analyses.

$($a$)$ What weight of the compound is required to make $1\mathrm{.}00$ dm$3$ of a $100$ ppm K standard?

$($b$)$ What volumes of this stock are required to prepare $100$ cm$3$ each of:

i a solution containing $10$ ppm NH$4+,$

ii a solution containing $25$ ppm of P$.$

$[$Hint for $($b$)$: work out first how many moles of salt were dissolved to make the solution in $($a$)$; hence for $($i$)$ calculate the ppm for NH$4+$ in the original stock, then use the equation for dilution:

$($ppm$)1$xV$1=($ppm$)2$xV