FORM III TOPIC 5. VOLUMETRIC ANALYSIS.

CHEMISTRY FORM THREE QUESTIONS AND ANSWERS 

FORM III TOPIC 5. VOLUMETRIC ANALYSIS.

QUESTIONS.
1. Identify the letter of the most correct answer.
   i)In a standard solution the following are known:-
      A: mass and moles  B: molarity and concentration
      C: amount of solute and solvent D: mass and molar mass

  ii)Apparatus for measuring accurate volume of liquids are:-
      A: pipette and burette  B: beaker and dropper
      C: Conical flask and reagent bottle D: measuring cylinder and funnel

 iii)The quantity of substance in volumetric analysis expressed in the following forms except:-
      A: number of moles present in certain volume of solution
      B: mass of solute dissolved to make known volume of solution.
      C: molarity
      D: number of particles

  iv)Both methyl orange and phenolphthalein indicators are not used when titrating
      A: weak base against weak acid  B: strong base against strong acid
      C: strong acid against alkaline solution. D: weak base against strong acid

  v)It involves weighing, preparation of solution and calculation
      A: quantitative analysis.  B: volumetric analysis
      C: chemical equations.    D: chemical kinetics

 vi)When water added to the basic compound, the mixture becomes:-
      A: heterogeneous  B: saturated. C: alkaline  D: unsaturated

 vii)Sources of errors when conducting titration experiments may include
       A: reading errors and timing burette tape for the last drop.
       B: mixing error
       C: noise
       D: impurities

 viii)Adding acid from burette into the conical flask containing alkaline solution with indicator is known as:-
      A: titration  B: standardization. C: emulsion  D: basicity

  ix)Any amount of solute dissolved in a solvent to make one litre of a solution is called
      A: molar solution. B: standard solution. C: stock solution  D: molarity

    x) The last drop of solution during titration that lead to the colour change indicates:-
       A: End point of the reaction. B: starting point of the reaction
       C: the reaction is not complete.  D: the solution added in excess.

2. Match list A with items in list B

    List A.                 
  i)Dropper.       
 ii)Burette.           
 iii)Funnel.
iv)Wash bottle
 v)Pipette
 vi)Retort stand
 vii)Conical flask
viii)White tiles
  ix)Measuring flask
  x)Weghing bottle

  A: measures volume of solutions especially base.

  B: Used to transfer and measure volume of solutions especially acids.

  C: measures accurate volume of liquid solutions.

  D: Add liquids especially indicators in form of drops.

  E:Transfers liquids into containers.

  F: Used as container when measuring mass of solutes.

  G: Holds burette during titration.

  H: Holds pipette during titration.

  I:  Gives clear back ground for observation.

  J:  Keeps distilled water.

  K: Used to mix acid and base when titrating.
  L: Measure specific volume when preparing standard solution.

3. Give the meaning of the following words(statements).
   a) Alkali
   b) Strong base
   c) Molarity
   d) Standardisation
   e) Standard solution
   f) Weak acid
   g) Neutralisation
   h) weak base
   i) Strong acid.

4. During titration methyl orange and phenolphthalein indicators used, state the suitable indicator when titrating.

   i) strong acid against strong base

   ii) weak acid against weak base

   iii) strong base against weak acid

   iv) weak base against strong acid

5. Why it is adviced to add acid into water but not water into a very concentrated acid?

6. Calculate mass of solid potassium chloride to prepare 250cm3 of 0.235M solution.

7. A solution of HCl contains 3.6g of the acid in 1dm3 of solution. 20cm3 of this solution is neutralized by sodium carbonate solution in which 1.1g of it dissolved to make 100cm3 of solution.

(a) Calculate mole ratio for the reaction.

 (b)Calculate the molarities of acid and carbonate solution.

(c) Calculate the volume of the carbonate solution used.

8. 14g sample of impure KOH was dissolved in water to make 100cm3 of solution. The solution was then titrated with 1.26M of nitric acid solution. 40cm3 of the acid  required 25cm3 of alkal solution to be neutralized. Work out the percentage purity of KOH sample.

(b) Which solution seem to be more concentrated for that neutralization reaction? State reason.

9. Washing soda has a formula Na2CO3•H2O
   7.15g of hydrated sodium carbonate dissolved in water to make 250cm3 of its solution. 25cm3 of this solution was titrated with 0.25M of hydrochloric acid. The end point reached when exactly 20cm3 of acid was used.

  Na2CO3 + 2HCl -----> 2NaCl + H2O + CO2.

(a) How the end point for the reaction was observed?

(b) What is the number of moles of the acid used?

(c) Calculate the concentration of the sodium carbonate in mole per litre.

(d) Find the formula molecular mass of hydrated sodium carbonate.

(e) Find molecules of water o crystallization of carbonate compound.

10. Samples of sodium hydroxide solutions of unknown concentration  were titrated against  0.2M of ethanoic acid and the following results obtained.
The volume of pipette used was 10cm3.

(a)  What is the average volume of the acid used

     (b) Calculate molarity of NaOH and its concentration in g/dm3 if the chemical equation is

 NaOH + CH3OOH--->CH3COONa + H2O

   (c) Name the suitable indicator for this reaction and state reason.

   (d) State change of colour observed during titration.

11.   48g of metal carbonate(Y2CO3) were dissolved to make 200cm3 of solution. The solution was titrated with 1M of sulphuric acid, 20cm3 of the solution reacted completely with 37.5cm3 of the sulphuric acid . Determine
   (a) the number of moles of the sulphuric acid used in the titration.
   (b) the number of moles in 20cm3 of the metal carbonate solution.
   (c) the R.A.M of the metal in metal carbonate.

3. a) Refers to the base that dissolves in water.

   b)  Is a base that dissociates completely to form ions.

      *more hydroxide ions formed when strong base dissociated(ionize)

   c) Refers to number of moles of solute dissolved in a solvent to make one litre of solution.

   d) Is a process of determining exact concentration of solution.

   e) Is a solution which its concentration is know.

      *amount of solute and solvent are known in standard solution.

   f) Is an acid that dissociates partially to form ions.

  *few hydrogen ions formed when weak acid ionize.

   g) Is a quantitative reaction between acid and base such than no excess hydrogen or hydroxide ions.

   h)  Is a base that dissociates partially to form ions.

   i) Is an acid that dissociates completely to form ions.

  iii) Phenolphthalein(POP)

5. Because when water added in a very concentrated acid, the solution boils very violently splashing acid out of container.

But when acid added into water no boiling and splashing occur.

    Concentrated acids are denser than water. Adding water into acid forms layer of boiling aced and splashing occurs.

      Adding acid into water, acid flow down and mixes much better with water. No boiling and splashing although the reaction remains exothermic.

      Given data: Molarity = 0.235M

                Relative atomic masses: K=39

     0.235 is number of moles in 1 litre.

           0.235 -----------> 1000cm3

    0.05875  is number of moles of KCl in 250cm3.

    Number of moles(n) = mass/molar mass.

       Mass=n x number of moles

7.(a)The mole ratio for the reaction obtained from the balanced chemical equation

   2HCl + Na2Co3 ------> 2NaCl + Co2 +H2O

The chemical equation shows that:-

     There are two HCl for every 1 NaCo3.

              Molar mass of HCl= (1+35.5)

 Molarity=Concentration(g/dm3)molar mass

Molarity of carbonate solution.

    Its molar mass=[(23 x 2) +12 + (16 x 3)

        Molarity=conc/molar mass

But concentration must be in g/dm3

Concentration of sodium solution is 11g/dm3

        Molarity of acid(Ma)=0.1M

        Molarity of base(Mb)=0.1M

        Volume of acid(Va)=20cm3

        Volume of base(Vb)= required

        Number of moles of acid(na)=2

        Number of moles of base(nb)=1

    Volume(KOH) solution=100cm3

    Volume of acid used=40cm3

    Volume of alkali used=25cm3

    %purity of KOH=required.

In order to get %purity, mass of pure substance and impure substance must obtained.

Mass of pure substance obtained from molarity that we get after titration.

   Mole ratio(na and nb) obtained from the balanced chemical equation.

   KOH + HNO3 ----> KNO3 + H2O

   2M(KOH) is a molarity of KOH made by pure sample of KOH.

 Mass(concentration)=molarity x molar mass

  112g is mass of pure KOH in 1000cm3

Mass of impure KOH is 14g in 100cm3

   140g is mass of impure KOH in 1000cm3.

 Mass of pure KOH/Mass of impure KOH x100%

(b) KOH was more concentrated because small amount of its volume used to neutralise acidic solution. It means the number of hydroxide ions in 25cm3 is equal to the number of hydrogen ions that found in 40cm of acidic solution.

9. (a) The end point for the reaction was observed by colour change of methyl orange from yellow to pink.

    Number of moles in 1000cm3=0.25moles

Number of moles of volume used(20cm3)=?

         ? <---------------- 20cm3="" p="">
The number of moles of acid used =0.005mol.

        Molarity of acid = 0.25M

        Volume of acid used=20cm3

       Molarity of base=required

       Volume of base used=25cm3

      Number of moles of acid=2

      Number of moles of base(n)=1

         Molarity(Na2CO3•H2O)=0.1M

         Its concentration = 7.15g in 250cm3

         Molar mass of Na2CO3•H2O=required

 In order to obtain molar mass. Concentration in g/L have to be determined.

   Molarity = concentration/molar mass.

Molar mass=concentration/molarity

         The molar mass(Na2CO3•H2O)=286g

         Number of molecules of water of

                   crystallization=required.

     (23 x 2) + 12 + (16x 3) + w((1x2)+16)=286

    w  stands for # of water of crystallization

 The number of molecules of water of crytall

   Average volume of the acid used obtained by taking sum of titre values of experiment 1, 2 and 3 divide by number of experiments.

   Titre value obtained by finding different between final burette reading and initial burette reading.

 Titre value for experiment:- 1  = 45.05-21.50

                           experiment  2 =30.00-10.05

                          experiment 3  =40.00-20.00

   Number of experiments is 3

Average volume(acid) = (20.05+19.95+20)/3

        Molarity of acid(Ma)=0.2M

        Volume of acid(Va)= 20cm3

        Molarity of NaOH(Mb)=required

        Volume of base (Vb)=10cm3

        Number of moles of acid(na)=1

        Number of moles of base(nb)=1

    (c) Suitable indicator is Phenolphthalein because the titration is between weak acid against strong base.

    (d) The colour change was from pink to colourless

                 Molarity of sulphuric acid = 1M

                 Number of moles(n) in 37.5cm3= required

                 Volume of sulphuric acid used=37.5cm3

       Molarity means number of moles of the substance which dissolved to make 1 litre of the solution.

   There fore 1M of sulphuric acid meana 1 mole of sulphuric acid found in 1 litre of the solution.

     1 mol -------> 1000cm3(1L)

      0.0375mol used in the reaction.

      (b)  The molarity of the metal carbonate(base) should be calculated first.

            Molarity of acid(Ma) =1M

            Molarity of base(Mb) = required

            Volume of acid = 37.5cm3

    H2SO4  +  Y2CO3 -----> Y2SO4 +  H2O +  CO2

   Number of moles of acid(na)=1

   Numner of moles of base(nb)=1

            1.875mol of metal carbonate dissolved to make 1 litre.

        1.875mol -------> 1000cm3

    0.0375mol found in 20cm3 of metal carbonate solution.

   (c) Concentration in g/litre should be calculated first.

                Molarity = concentration/molar mass

        Whereby molarity = 1.875M

                        Molar mass = (2Y+ 12 + (16 x 3))

       Concentration should be:-

      Molarity = concentration/molar mass

      Molar mass= concentration/molarity