helllpp (2 Viewers)

[~GrOoVy~]

okay while i was revising for my 1\2 yearlies i stumbled upon some tricky questions. Can someone here plzzzz solve them for me

1. which of the following exhibits coordinate covalent bondings?
a. oxygen molecule
b. hydronium ion
c. ammonia
d., ethanol
(i would also like to know what bonding the other's possess.)

2. A student used a pipette to transfer 25 ml of a solution to a flask. After draining the solution into the flask a small amount of solution remained in the tip of the pipette. To deliver the correct volue of solution to the flask the student should:
a. blow the remaiing solution into the flask
b. touch the inside of the flask with the tip of the pipette
c shake the pipette to dislodge the remianing solution

3. to prepare a standard solution of sodium hydroxide a student first dissolved 1.0g of solid sodium hydroxide in 250ml of distilled water. By titration 25.0ml of this solution required 23.2ml of standard 0.100mol L^-1 HCL for neutralisation.

a. why is titration neccessary to standardise the sodium hydroxide solution?

oh and one more thing is it neccessary to learn about the standard hydrogen electrode?????


thats alll...thanks in advance for your help

 

[physician]

well for question 2... a pipette is actually made so that the remaining solution at the tip is not part of the volume... as in.. with the ramining portion at the tip.. u'd still have 25ml.. and so if u did do (a,b or c)... u would increase the amount to 25+ml .. making it inacurate...

I could be worng... but i'm pritty sure i've read that in at least 2 books... and when we peformed a titration experiment as an asessment... the remaining solution in the tip was to be ignored as u would have 25ml in the falsk...

 

[shannonm]

1. is hydronium ion
ammonia can form ammonium nh4+ which also exhibits ccb

2. i agree with the dude above

3. titration is necessary because its one of the easiest ways to standardise a soln. ?



oh and yeah you need to know standard electrode potentials, just remember 2H+ + 2e- <-> h2 is the reference (treated as 0V) i dont think you need to know the actual setup (platinum, sulphuric aid, blah)

 

[xiao1985]

agree with above 2 posts on q1,2

q3: cuz sodium hydroxide is not a primary standard... it need to be standardised as its concentration maybe vary by absorbin water in present in the air...

i highly doubt u need to know that hydrogen standard electrode... but u do need to know that the potential table is derived by using oxidation and reduction of hydrogen molecule/ion as a standard at 0V... the other voltage is the voltage obtained by comparing to this standard...

 

[lfc_reds2003]

these dudes all know wat they are on about
lol

 

[Templar]

For question 2, in the absence of the answer of do nothing, it should be b, touch the inside with the tip.

 

[physician]

For question 2, in the absence of the answer of do nothing, it should be b, touch the inside with the tip.

yes! but that would not be delivering the correct volume into the flask.. as it increases the volume... therefore resulting in an error.. and as we know titration is supposed to be a very accurate procedure

by the way ~GrOoVy~ for question 2 ... are u sure there is no alternative ( d )?.. as the HSC questions have 4 alternative answers...

 

[~GrOoVy~]

thanks for answering the question guyz

physician: yeah there was a (d) but i it was about washing the remaining out with distilled water...

 

[~GrOoVy~]

1. which of the following exhibits coordinate covalent bondings?
a. oxygen molecule
b. hydronium ion
c. ammonia
d., ethanol

okay i still don't really get this question. Like how would u know if a substance exhibits coordinate covalent bondings???

thanks

 

[funking_you]

you have to draw the lewis electron dot-structures for each.

The hydronium ion is H₃O⁺

Draw the lewis diagram for water, and you will notice that the oxygen atom has a pair of non-bonding electrons, and since the H⁺ ion has no electron, the 'new' or extra H-O bond is a coordinate covalent bond, where those two non-bonding electrons from the oxygen form the new covalent bond with the H⁺ ion.

 

[Sirius Black]

yes! but that would not be delivering the correct volume into the flask.. as it increases the volume... therefore resulting in an error.. and as we know titration is supposed to be a very accurate procedure

by the way ~GrOoVy~ for question 2 ... are u sure there is no alternative ( d )?.. as the HSC questions have 4 alternative answers...

It is definitely (b)-touching the tip-if u actually did the titration, u would know the last drop won't come down if the pipette touches the flask and u count for 1,2,3 and then take the pippete out... that's what i was told by a chem PhD student

 

[physician]

It is definitely (b)-touching the tip-if u actually did the titration, u would know the last drop won't come down if the pipette touches the flask and u count for 1,2,3 and then take the pippete out... that's what i was told by a chem PhD student

Fair enough... I may have stumbled across something like that... but the reason we didn't have to do anything like that was because u allready have the correct volume... and the question states "To deliver the correct volume of solution to the flask"... But very interesting...

Maybe i just stopped reading at the point where it said "ignore the ramaining solution at the tip of the pipette"... but it's good that u cleared that up incase it does get asked in an examination...

 

[nit]

Question 2 is definitely b.) - you drain some of the remaining solution from the pipette by resting the pipette on the side of the conical for a fixed period of time - eg you rest it there for 2/3 seconds each time you titrate in order to make the procedure as reliable as possible.

 

[shafqat]

id trust nit
he's a chemistry olympian

 

[~GrOoVy~]

....stuck again

 

[lilkatie]

yeah i agree i'd go with b
but yeah aren't those things configured to account for bit left in bottom......hmm just a weirdo q

 

[~*HSC 4 life*~]

you do need to know the standard hydrogen electrode, it's a syllabus dot point! just know
- its purpose (allocated the E0 value of 0.00V so it could be used as a standard to measure other E0 values )
- what it is made of (platinum electrode in a 1M H+ solution with Hydrogenhas bubbled through i think from memory)
- how it works: (this half cell is usually hooked up to another half cell, it is designnated the E0 value of 0.00V, so anything that oxidises is given a positive E0 value, and anything that reduces is given a negative E0 value)
- the conditions in which it operates: temperature, molarity of h+ solution and pressure)

i HAVE seen past trial exam paper questions where you need to know the above

 

[xiao1985]

nothing's gonna happen...

so4 2- is a terrible base
na+ is a terrible acid...

 

[~*HSC 4 life*~]

nothing's gonna happen...

so4 2- is a terrible base
na+ is a terrible acid...

really? i'm not sure but i dunno i went with A...cause you're adding sulfate anyway to the system, wouldn't it shift left according to LCP anyway?...not sure tho...hmm i get your point about sulfate being a bad proton acceptor but i dont getw hy it wont change anything...

 

[m_isk]

yeh nothing will happen. THis is because in acid/base reactions as such, it is the acid/base SOLUTION which reacts, they don't react as solids. eg if u used litmus paper with a solid acid (i.e no water) it would not change colour. Why? there's no soln. so the answer is C