(i) Define the terms strong acid and weak acid. Using hydrochloric
    and ethanoic acid as examples, write equations to show the dissociation
    of each acid in aqueous solution.
    ……………………………………………………………………………………………
    ……………………………………………………………………………………………
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Ans  :    strong acid completely dissociated/ionized;
            weak acid only partially dissociated/ionized;
            HCl(aq) <---->  H+(aq) + Cl+(aq);
            CH3COOH(aq)<---> CH3COO–(aq) + H+(aq);   
            Insist on both arrows as shown, state symbols not needed.
            Also accept H2O(1) and H3O+(aq) in equations


(ii) Propanoic acid, CH3CH2COOH is a weak acid.
    (a)    Give the equation for the ionization of propanoic acid in water
            and deduce the expression for the ionization constant, Ka, of
            propanoic acid.
            ……………………………………………………………………………………….
            ……………………………………………………………………………………….
            ……………………………………………………………………………………….
    (b)    Calculate the Ka value of propanoic acid using the pKa value in the
            Data Booklet.
            ……………………………………………………………………………………….
            ……………………………………………………………………………………….
    (c)    Use your answer from (b) to calculate the [H+] in an aqueous
            solution of propanoic acid of concentration 0.0500 mol dm–3,
            and hence the pH of this solution.
            ……………………………………………………………………………………….
            ……………………………………………………………………………………….
            ……………………………………………………………………………………….
           ……………………………………………………………………………………….
            ……………………………………………………………………………………….
            ……………………………………………………………………………………….

Ans : (a)    CH3CH2COOH   CH3CH2COO– + H+ (no penalty for  );
                Accept equation with H2O and H3O+.   
                Ka =  ;
        (b)    1.35×10–5 (mol dm–3)    1
                Units not needed. No penalty for too many sig figs.
        (c)    [H+] = 
                       =  8.21 or 8.22×10–4 (mol dm–3);
                Units not needed, ECF from (b). No penalty for too many sig figs.
                pH (= –log[H+] = –log (8.22×10–4) = 3.1 or 3.09 (ECF) penalize
                 more than 4 sig figs;
                If expression for [H+] missing but both answers correct,
                award [3],
                if one answer correct, award [2].


(iii)  Predict whether each of the following solutions would be acidic,
      alkaline or neutral. In each case explain your reasoning.
    (a)     0.1 mol dm–3 FeCl3(aq)
    …………………………………………………………………………………………..
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    (b)    0.1 mol dm–3 NaNO3(aq)
    …………………………………………………………………………………………..
    …………………………………………………………………………………………..
    …………………………………………………………………………………………..
    (c)    0.1 mol dm–3 Na2CO3(aq)
    …………………………………………………………………………………………..
    …………………………………………………………………………………………..
    …………………………………………………………………………………………..
Ans : (i)    acidic;
                Fe(H2O)63+ is a weak acid/Fe3+ reacts with OH–/equation
                to show formation of HCl or H+;
                “FeCl3 is acidic” is not acceptable. 
        (ii)    neutral;
                NaNO3 / sodium nitrate is formed from strong base and strong
                acid/ions do not hydrolyse;  
        (iii)    alkaline;
                As CO32– is weak base/combines with H+/equation showing
                formation of OH–;    2
                Acidic, neutral, alkali mark in each case is independent of reason.