Conjugate acid-base pairs

What is the conjugate acid of the hydrogen phosphate ion, $\ce{HPO4^2-}$? A. $\ce{PO4^3-}$ B. $\ce{H2PO4-}$ C. $\ce{H3PO4}$ D. $\ce{HPO3^2-}$

Which is a conjugate acid-base pair? A. $\ce{H2CO3}$ / $\ce{CO3^2-}$ B. $\ce{H2SO4}$ / $\ce{SO4^2-}$ C. $\ce{HSO4-}$ / $\ce{SO4^2-}$ D. $\ce{CH3NH3+}$ / $\ce{CH3NH-}$

Methanoic acid, $\ce{HCOOH(aq)}$, and hydrochloric acid, $\ce{HCl(aq)}$, are both acids. A $0.100\ \mathrm{mol\,dm^{-3}}$ solution of $\ce{HCOOH(aq)}$ has a pH of 2.39 while a solution of $\ce{HCl(aq)}$ of the same concentration has a pH of 1.00. (i) Explain…