2019 NECO CHEMISTRY PRC EXPO

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(1)

The volume or pipette used 25.00cm³

In a tabular form 

Titrations |rough |1st|2nd|3rd|

Final reading |21.00|41.30|20.40|40.90

Initial reading|0.00|21.00|0.00|20.40

Vol of A used |21.00|20.30|20.40|20.50

(1ai) 

Average volume of A = 20.30+20.40+20.50/3

=61.2/3

=20.40cm³

(1aii) 

H2X(aq) + 2NaOH(aq)-->Na2X(aq) + 2H2O(l)

(1bi) 

Mass conc. of B = 1g/250cm³

= 1g/0.25dm³ = 4g/dm³

Using :

Molar conc. = mass conc./molar mass

= 4/(23+16+1) = 4/40 = 0.1mol/dm³

(1bii) 

Using CAVA/CBVB = nA/nB

CA = CBVBnA/VAnB

CA = 0.1×25×1/20.4×2

Concentration of A = 0.06127mol/dm³

(1biii)

Molar mass of A = mass conc/molar conc

=4.85/0.6127

=79.15

≅ 79gmol

(1c)

(i)it might contain traces of impurities which must be removed before being weighed 

(ii)it is hygroscopic which readily absorb moisture from atmosphere

*ANSWERS TO Q2 NECO PRATICAL CHEMIRTRY*

== = = = = = = = = = = = = == = = = =

*1st line*:|test|:Xn +distilled water,|observat

ion|:Xn dissolves in distilled water,|inference|:Xn is a soluble salt

= = = = = = = = = == = = = =

*2nd line*:|test|:1st portion+NAOH<aq> in drops and in excess,|observation|:pale blue ppt,|inference|:CU2+

= = = = = = = = == = = == =

*3rd line*:|test|:2nd portion+NH3 in drop and in excess,|observation|:deep blue ppt,|inference|

:CU2+

= == = = = = = == == = = = =

*4th line*:|test|3rd portion+BACL2,|

observation|:white ppt,|inference|

:SO42-,SO32-,CO32-,CL-

= = = = = = === = == = = == =

*5th line*:|test|:+ dil HCL,|observation|:insoluble in HCL,|Inference|:SO42-

Fill in the space.

(2i)

Observation;

Solution turns blue

Inference;

Copper salt suspected.

(2ii)

Observation;

Blue gelatinous precipitate

Insoluble in excess NaOH

Inference;

Cu²+ suspected

Cu²+ Confirmed

(2iii)

Observation;

Blue gelatinous precipitate

Soluble in excess NH³

Inference;

Cu2²+ Suspected 

Cu²+ Confirmed 

(2iv)

Observation;

White precipitate 

Inference;

SO4²- Suspected 

(2v)

Observation;

Precipitate insoluble in excess 

Inference; 

SO4²- confirmed

3ai)  methyl Red

 3aii) * Red

        * Colourless

3aiii) To give a sharp change in colour at the end of the acid -base reaction

3aiv)

 i) the burette must be rinsed with the acid or allow it to drain after rinsing with distilled water

  

ii) Air bubble must not be allowed in the burette

3v)  neutralization reaction

3bi)  hopes Apparatus 

3bii)  kips apparatus

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