Monday, October 22, 2007

Chemistry Question - Acids,Bases & Salts, Organic Chem


Q1) desribe an expt that you wld caryy out t show that sulphuric acid is a strong acid but ethanoic acid is a weak acid

Ans: This is testing you on the properties of acid. There are 4 ways for you to differentiate acid - pH paper test, Reaction w Metal, Reaction with Carbonate or Reaction with an Alkali. To differentiate the acidity of Sulphuric Acid and Ethanoic Acid, i would suggest to test the amount of CO2 Gas produced when certain amount of the acid are reacted with a certain mass of say calcium carbonate. Suphuric Acid, being stronger acid will tends to give off CO2 gas more and at a faster rate.

Q2)2 salts formed frm sodium hydroxide and phosphoric acid other than NaH(2)PO(4.

Ans: Phosphoric Acid is TRIBASIC, meaning it has 3 Hydrogen atoms to dissociate. It will form 3 salts when reacted with NaOH.

H3PO4 -> 3H+ + PO4^3-
H3PO4 -> 2H+ + HPO4^2-
H3PO4 -> 1H+ + H2PO4-

NaOH + H3PO4 -> Na3PO4 (Ans) + H20
NaOH + H3PO4 -> Na2HPO4 (Ans) + H20
NaOH + H3PO4 -> NaH2PO4 + H20

PS: Pls not equestion is not balanced yet.

Q3) explain why NaH(2)PO(4) is both an acid and a salt.
Ans: It is an acid bcos it can dissociates H+ in water making the solution acidic.
It is a salt bcos it is made by the neutralization process of NaOH and H3PO4. Anyhow, it can also be made by the reaction of H3PO4 with Na metal or NaCO3.

Q4) structural formula f repeat unit of styrene-butadiene
Ans: This should not be covered in GCE O level syllabus. Learnt this poly/uni.
You should know the structural formulae of Styrene monomer, but the question should give you the structural formulae of butadiene. With those structures, use the theory of Addition Polymerization.

All The Best!
Learning Experience with A Difference


illuminate-x said...

THANK YOU SO MUCH for the tip for the weak/strong acid question! It came out for Chem Paper 2 today and I was so relieved I saw the post just yesterday. :)

Anonymous said...

oh my. is it ok if i say i'll use a pH meter? =/

Anonymous said...

should be ok one. last time my sch paper came out i wrote about ph paper.
they did not limit it just to chemical/physical test :)