(i) $\ce{HCl}$ is a strong acid and ionizes/dissociates completely, so [$\ce{H+}$] equals the acid's concentration «0.100 mol dm$^{-3}$» ✔

$\ce{HCOOH}$ is a weak acid and only partially ionizes «an equilibrium is set up», so [$\ce{H+}$] is lower than the acid's concentration, giving a higher pH ✔

(ii) both acids react «completely, since Mg is in excess» to give the same final volume of gas «and the same temperature rise», because both solutions contain the same amount, in mol, of acid ✔

bubbles of gas form faster / more vigorously with $\ce{HCl}$ than with $\ce{HCOOH}$, because the initial [$\ce{H+}$] is higher in the strong acid ✔

(iii) conjugate acid $\ce{HCOOH}$«(aq)» AND conjugate base $\ce{HCOO-}$«(aq)» ✔

conjugate acid $\ce{H3O+}$«(aq)» AND conjugate base $\ce{H2O}$

(iv) +2 ✔

(i) M1: HCl fully ionizes so [H+] = concentration; M2: HCOOH only partially ionizes so [H+] is lower, giving a higher pH. (ii) M1: same final volume of gas / amount of product, because equal moles of acid are present; M2: faster / more vigorous reaction with HCl because of its higher initial [H+]. (iii) [1] for each pair; both members of the pair are needed for the mark. (iv) [1] for +2; accept (II); do not accept 2+ or 2.