My Chemistry Thread

[Yacoubb]

So if you were asked to draw the structural formula of benzene, and you had never seen how it looked like before, what would be your procedure of doing so? I don't understand why it is the way it is, and not the other cyclic version (previously assumed to be correct) which has all carbons linked by double bonds.

Benzene is an aromatic compound, not a cyclic alkene. So, I'd draw 6 carbons with interchanging double bonds (i.e. one double bond and then one single bond), with one hydrogen per carbon.

[alchemy]

Quick question: Are more polar molecules (that have a greater electronegativity difference) stronger or weaker than lesser polar molecules (that have a lower electronegativity difference)? Why is it so?

[Limista]

Quick question: Are more polar molecules (that have a greater electronegativity difference) stronger or weaker than lesser polar molecules (that have a lower electronegativity difference)? Why is it so?

If you've got 2 polar molecules, you know that both of them have opposite electrostatic attractions. Now you just have to compare the magnitude of these electrostatic attractions.

This means that a polar molecule with a dipole of -5 at one end, and a dipole of +5 at the other end, will be more compact, because the dipoles are more strongly attracted to each other compared to the other polar molecule with +3 and -3 or +2 and -2.

[alchemy]

If you've got 2 polar molecules, you know that both of them have opposite electrostatic attractions. Now you just have to compare the magnitude of these electrostatic attractions.

This means that a polar molecule with a dipole of -5 at one end, and a dipole of +5 at the other end, will be more compact, because the dipoles are more strongly attracted to each other compared to the other polar molecule with +3 and -3 or +2 and -2.

Oh right, so is this sufficient to explain why H₂O has a higher boiling point than HF, even though both are polar and undergo hydrogen bonding? Or is it because each H₂O molecule has a greater dispersion force, than that of HF, because of its larger atomic radius? I've heard, however, that you shouldn't refer to dispersion forces unless they are the major force of attraction. In this case, I'm not sure if they are...

[Limista]

Oh right, so is this sufficient to explain why H₂O has a higher boiling point than HF, even though both are polar and undergo hydrogen bonding? Or is it because each H₂O molecule has a greater dispersion force, than that of HF, because of its larger atomic radius? I've heard, however, that you shouldn't refer to dispersion forces unless they are the major force of attraction. In this case, I'm not sure if they are...

To explain why H2O has a higher boiling point than HF, you'd need to refer to the specific heat capacity.

In this case, the polarity doesn't have much to do with it.

[alchemy]

An accurate measurement of the mass of an unknown compound gave a result of 260.0429. What is
the formula of the compound?
Atomic masses: C = 12.0000, H = 1.0078, N = 14.0030, O = 15.9949
a. C13H8O6
b. C12H8N2O5
c. C8H8N2O8
d. C9H14N3O6
e. all of the above

The stating of Atomic masses I find slightly ambiguous. Are they referring to the general known masses of those atoms which are found on the Periodic Table as well, or are they referring to the masses of those atoms actually in the unknown compound?
I'm usually good at these questions but my head isn't working at the moment...uhh!

[IndefatigableLover]

An accurate measurement of the mass of an unknown compound gave a result of 260.0429. What is
the formula of the compound?
Atomic masses: C = 12.0000, H = 1.0078, N = 14.0030, O = 15.9949
a. C13H8O6
b. C12H8N2O5
c. C8H8N2O8
d. C9H14N3O6
e. all of the above

The stating of Atomic masses I find slightly ambiguous. Are they referring to the general known masses of those atoms which are found on the Periodic Table as well, or are they referring to the masses of those atoms actually in the unknown compound?
I'm usually good at these questions but my head isn't working at the moment...uhh!

Forgive me if I'm totally wrong (not exactly the brightest at Chemistry LOL) but when they mention the word 'accurate' and then state the atomic masses.. I think they're referring to the masses of the atoms in the unknown compound rather than the general known ones because it's not really 'accurate' (well enough for it to be general but you get what I mean I guess?).
I'd assume for this question you'd just plug in values for those formulae and you'd get your answer right (if that's it then it should be 'B' right?)

[alchemy]

Forgive me if I'm totally wrong (not exactly the brightest at Chemistry LOL) but when they mention the word 'accurate' and then state the atomic masses.. I think they're referring to the masses of the atoms in the unknown compound rather than the general known ones because it's not really 'accurate' (well enough for it to be general but you get what I mean I guess?).
I'd assume for this question you'd just plug in values for those formulae and you'd get your answer right (if that's it then it should be 'B' right?)

Oh right, fair enough. I guess the key word was "accurate". In fact if you just round off those figures and plug them into every one of the given formulas of the compound, you'd get 260. But you'd have to plug in the decimal numbers with a calculator to get 260.0429. Makes sense now, thanks!

[alchemy]

An equation in the book for the reaction of an acid with a metal hydroxide states:
H2SO4(aq) + 2NaOH(aq) —> Na2SO4(aq) + 2H2O(l)
Why is NaOH indicated as being dissolved in water? I thought NaOH is insoluble?


EDIT: Ignore question, false info provided. NaOH is actually soluble in water.. my bad!

[Rishi97]

An equation in the book for the reaction of an acid with a metal hydroxide states:
H2SO4(aq) + 2NaOH(aq) —> Na2SO4(aq) + 2H2O(l)
Why is NaOH indicated as being dissolved in water? I thought NaOH is insoluble?

I'll have a try: This is probs going to be wrong so pls correct me someone.
NaOH is soluble in water because it has an OH group which forms hydrogen bonds with water thus making it soluble 
Forgive me if I'm wrong

[alchemy]

I'll have a try: This is probs going to be wrong so pls correct me someone.
NaOH is soluble in water because it has an OH group which forms hydrogen bonds with water thus making it soluble 
Forgive me if I'm wrong

Oh turns out it is soluble... I checked my solubility table just now. Sorry for the confusion guys. Thanks for making me realise that Rishi!

[Rishi97]

Oh turns out it is soluble... I checked my solubility table just now. Sorry for the confusion guys. Thanks for making me realise that Rishi!

No probs
Glad you got it now

[lzxnl]

To explain why H2O has a higher boiling point than HF, you'd need to refer to the specific heat capacity.

In this case, the polarity doesn't have much to do with it.

Actually, it's because for every water molecule, you have two hydrogens which can form H bonds, as well as two lone pairs on the oxygen, which can also H bond. Therefore, in water, there is the maximum amount of hydrogen bonding possible in which every atom is H bonded to another. Hence its higher boiling point than HF. HF has one H per molecule and 3 lone pairs per F; see the difference?

An equation in the book for the reaction of an acid with a metal hydroxide states:
H2SO4(aq) + 2NaOH(aq) —> Na2SO4(aq) + 2H2O(l)
Why is NaOH indicated as being dissolved in water? I thought NaOH is insoluble?


EDIT: Ignore question, false info provided. NaOH is actually soluble in water.. my bad!

Sodium salts are generally soluble. Assume all sodium salts in VCE are soluble.
Hydroxides, however, generally aren't. I don't know of a single soluble transition metal hydroxide, for instance.

[IndefatigableLover]

Sodium salts are generally soluble. Assume all sodium salts in VCE are soluble.
Hydroxides, however, generally aren't. I don't know of a single soluble transition metal hydroxide, for instance.

Just going to chime in but I thought for VCE you could assume that all Group One Metals (or salts) are soluble and although hydroxides are generally insoluble, when it is paired with a Group One metal then it is soluble?

[alchemy]

1.000 mL of 0.1000 mol L–1 hydrochloric acid was diluted to 100.0 mL with deionised water. 10.00
mL of this solution was diluted to 100.0 mL again using deionised water. What is the pH of the final
solution?

I was able to get the correct answer, but can somebody please let me know if my method here is correct? And if there is a shorter way to do the question, as the last few lines of working out sort of indicate that there might be?

My working:
n=c*v=0.1*(1/1000)=0.0001 mol
0.0001=c*100/1000
c=0.001M
Per litre there's 0.001 moles
Therefore, in 10mL there's 0.00001 moles   <--- Is this a legitimate statement to make?
0.00001 = c*100/1000
c=0.0001M
pH = -log₁₀[H⁺]=-log₁₀(0.0001)=4.