Chemistry OCR F321 past paper questions and answers on acids

1.       A student carries out experiments using acids, bases and salts.

          Calcium nitrate, Ca(NO3)2, is an example of a salt.

          The student prepares a solution of calcium nitrate by reacting dilute nitric acid, HNO3, with the base calcium hydroxide, Ca(OH)2.

(i)      Why is calcium nitrate an example of a salt?

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1.       (i)      The H+ ion in an (nitric) acid has been replaced by a metal ion
OR by a Ca2+ ion 

DO NOT ALLOW it has been produced by the reaction of an acid and a base as this is stated in the question.

IGNORE references to replacement by NH4+ ions or positive ions.
ALLOW H OR Hydrogen for H+;
DO NOT ALLOW Hydrogen atoms
ALLOW Ca OR Calcium for Ca2+.
DO NOT ALLOW Calcium atoms
ALLOW ‘metal’ for ‘metal ion

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(ii)      Write the equation for the reaction between dilute nitric acid and calcium hydroxide. Include state symbols.

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(ii)     2HNO3(aq) + Ca(OH)2(aq) → Ca(NO3) 2 (aq)+ 2H2O(l)
Formulae
Balance AND states 

ALLOW multiples
ALLOW (aq) OR (s) for Ca(OH) 2

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(iii)     Explain how the hydroxide ion in aqueous calcium hydroxide acts as a base when it neutralises dilute nitric acid.

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(iii)    Accepts a proton OR accepts H+ 

ALLOW H+ + OH–→ H2O
ALLOW OH– reacts with H+ OR OH– takes H+
ALLOW OH– ‘attracts’ H+ if ‘to form water’ is seen

DO NOT ALLOW OH– neutralises H+ (‘neutralises’ is in the question)

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(a)     A student carries out a titration to find the concentration of some sulfuric acid.

          The student finds that 25.00 cm3 of 0.0880 mol dm–3 aqueous sodium hydroxide, NaOH, is neutralised by 17.60 cm3 of dilute sulfuric acid, H2SO4.

                             H2SO4(aq) + 2NaOH(aq) → Na2SO4(aq) + 2H2O(l)

(i)      Calculate the amount, in moles, of NaOH used.

answer = ................................... mol

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(a)     (i)      Calculate correctly (0.0880 x 25)/1000 = 2.20 x 10–3 

OR 0.00220 mol 

ALLOW 0.0022 OR 2.2 × 10–3 mol

(ii)     Determine the amount, in moles, of H2SO4 used.

answer = ................................... mol

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(ii)     Calculates correctly  0.00220/2= 1.10 × 10–3 mol

OR 0.00110 mol 

ALLOW 0.0011 OR 1.1 × 10–3 mol

ALLOW ECF for answer (i)/2 as calculator value or correct rounding to 2 significant figures or more but ignore trailing zeroes

(iii)     Calculate the concentration, in mol dm–3, of the sulfuric acid.

answer = ................................... mol dm–3

(iii)    (0.00110 x 1000)/ 17.60 = 0.062 mol dm–3
OR 6.25 × 10–2 mol dm–3 

ALLOW 0.063 OR 6.3 × 10–2 mol dm–3

ALLOW ECF for answer (ii) × 1000/17.60
OR
ECF from (i) for answer (i)/2 × 1000/17.60 as calculator value or correct rounding to 2 significant figures or more but ignore trailing zeroes

(b)     After carrying out the titration in (a), the student left the resulting solution to crystallise. White crystals were formed, with a formula of Na2SO4•x H2O and a molar mass of 322.1 g mol–1.

(i)      What term is given to the ‘•x H2O’ part of the formula?

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(b)     (i)      (The number of) Water(s) of crystallisation 

IGNORE hydrated OR hydrous

(ii)     Using the molar mass of the crystals, calculate the value of x.

answer = ...................................

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(ii)     142.1 

ALLOW 142

ALLOW Mr expressed as a sum

ALLOW ECF from incorrect Mr and x is calculated correctly

         x =  (322.1 - 142.1)/18.0 = 10 

ALLOW ECF values of x from nearest whole number to calculator value

ALLOW 2 marks if final answer is 10 without any working

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