WAEC GCE Nov/Dec 2018 Chemistry Practical Expo Answer

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(1ai)

Tabulate:

Burette reading |1 |2 |3 |

final reading |24.80|34.50|20.65|

initial reading |0.00 |10.50|6.55 |

vol of acid used|24.80|24.00|24.10|

(1aii) Average volume of A used

Va = 24.00 + 24.10/2

= 24.05cm^3

(1bi)

Concentration of A in moldm^-3

A contains 0.79g of KMnO4 per 250cm³ of solution.

Hence since 250cm³ = 0.79g

100cm³ = Xg

X = 1000×0.79/250 = 3.16g

Hence Conc in g/dm³ of A = 3.16gdm^-³

But molar conc = mass conc(gdm^-³)/molar mass(g/mol)

Hence molar Conc.Of A= 3.16gdm^-3/molar mass ofA

But molar mass of A = KMnO4

= 39 + 55 + 4(16) = 39 + 55 + 64

= 158g/mol

Hence Conc in moldm^-³ of A = 3.16gdm^-3/158g/mol

= 0.020moldm^-3

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2)

A is ZnSO4(aq)

B is NaOH(aq)

C is Pb(NO3)2 (aq)

D is HCL(aq)

Reasons

ROW 1

it is used to distinguish Pb from Zn since both behave alike with NAPth

It gives a white precipitate with Pb^2+ ions but not with Zn^2+. The first

row obviously shows that. Also there actually no visible reaction whne Itcl (A) and NaOlt are mixed(C)

Row 2

when A and B mirces, the products Zncl2 and

H2SO4 one soluble hence no visible reaction

is scene Between B and D, a white precipitate is seen because PbaS04 insoluble

Between B and C, Zn^2+ On yield a white gelatinous precipate with NaOlta

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Question 3

a(i) Sodium is stored under paraffin oil to prevent its oxidation by atmospheric gases

(ii) desiccator lid must be greased with a thin layer of grease, to ensure an airtight seal.

(iii)

b(i)

(ii) Dip a rod in aqueous Ammonia and dip in a beaker containing the acids, only HCl produces dense white fumes of ammonium chloride

c(i) Beaker, glass rod, volumetric flask

(ii) A gas which smells like that of burning(SO2) matches evolves

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2)

A is ZnSO4(aq)

B is NaOH(aq)

C is Pb(NO3)2 (aq)

D is HCL(aq)

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