$9-26.$ Consult Appendix $3$ and pick out a suitable acid$/$ base pair to prepare a buffer with a pH of

$*($a$)3\mathrm{.}5.$

$($b$)7\mathrm{.}6.$

$*($c$)9\mathrm{.}3.$

$($d$)5\mathrm{.}1.$