Homework questions thread

[Hellrocks]

H3A3 + 3NaOH => 3NaA + 3H2O

n(NaOH)=CV=0.55x29.5E-3 mol
n(H3A3)=1/3n(NaOH)=5.41E-3 mol

C(H3A3)=n/V=5.41E-3/25E-3=0.2163M

0.2163M implies 0.2163 moles in 1L, I want the units to be grams in 1L.
Use n=m/Mr
m=nxMr=0.2163x192=41.536g in 1L

Ok thanks guys, i'm still abit unclear about the triprotic part though, and how u know the mol of citric acid is 3 times the mol of naoh.
@ Hellrocks, whats the H and A represent in ur eqn?

EDIT: Oh wait, it think i get it...but if possible to elaborate, please do so. Thanks

H is the acidic proton and A- is the conjugate of the weak acid. You will see HA next semester that represents a monoprotic weak acid. Sorry, at the time I didn't realise this wasn't covered in unit 3.

[Hellrocks]

Which of the following increase(s) for the bonding between carbon atoms in the sequence of molecules C2H6, C2H4 and C2H2?
1. Number of bond
2. Length of bonds
3. Strength of bonding

Answer is 1 and 3.
I just don't understand how you would know whether or not the length of bonds increase or decrease?

Double bonds are shorter than single, triple bonds shorter than double, and therefore by extension single.

Oh ok. Is it because stronger bonds have stronger forces of attraction, so pull the carbon atoms closer,hence shorter length as number of bonds increase?

You are absolutely correct. Increase in the number of bond increases the strength of the bond between 2 atoms. But it is important to realise a double bond is not twice as strong as a single bond nor is it half as long as a single bond between the same atoms.

[ecvkcuf]

What are all the errors that can occur in a volumetric and gravimetric analysis?
And what effect do they have on the results being analysed?


Also is there any other simpler way of finding the number of isomers of a hydrocarbon, or do I just have to draw it it out.. when drawing it out how do you know when you've reached the limit of number of isomers of that element..

[soccerboi]

Also is there any other simpler way of finding the number of isomers of a hydrocarbon, or do I just have to draw it it out.. when drawing it out how do you know when you've reached the limit of number of isomers of that element..

With isomers, i just try to draw them all out, but if anyone has any others ways which are more efficient, i would also like to know.

[soccerboi]

If given a mass spectrum and the highest m/z value is 74, and i've figure that the empirical formula is C3H6O2(74 g mol^-1), and the question asks for the molecular formula of this compound, can i state it as C3H6O2 or must it be more precise such as CH3CH2COOH?

[charmanderp]

C3H6O2 is a molecular formula, whilst CH3CH2COOH is a semi-structural formula. The question asks for the molecular formula, hence give the former.

[charmanderp]

Also is there any other simpler way of finding the number of isomers of a hydrocarbon, or do I just have to draw it it out.. when drawing it out how do you know when you've reached the limit of number of isomers of that element..

Just draw as many isomers as you can (considering things like methyl branches, placement of substituents, placement of functional groups (including double/triple bonds), etc) and then name them all at the end, to make sure you haven't doubled up.

[charmanderp]

Should I write states for every equation, or only if I'm asked for a 'balanced equation' or 'full equation'? My dilemma is when the question asks only for an equation and I'm not sure about the states. Do I put them there, potentially incorrectly, and run the risk of losing a mark when the states weren't even required?

Also, for mass spectrometry equations, do I write the states? It occurs in the gas state, of course...

Finally, a potentially silly question, but I'm desperate for affirmation. Is using semi-structural formula interchangeable with using molecular formula in the case of writing equations and mass spec fragments, when it's not specified in the question which form is required? This is in reference to organic compounds - C4H10O rather than CH3CH2CH2CH2OH, for example, or C2H5+ rather than CH3CH2+? Obviously semi-structural formula is more specific and removes ambiguity as to exactly which isomer you are referring too, but it's so messy! And it takes up room.

[ggxoxo]

For Mass Spec, TSFX said yes, put states in

[Destiny]

... TSFX is suffering internal divisions.  My teacher (works at TSFX - Chemistry) says different stuff to Irene. -.-

[soccerboi]

What fragment must have been lost from ethanoic acid to account for the peak at m/z 45?

I wrote CH₃⁺, but the answers just have it as CH₃.
So am i wrong to put the plus there and why?

[Destiny]

Must be a free radical (signified by the dot) lost as only charged ions hit the detector (shown by the peak).

[soccerboi]

Write an equation showing the formation of the fragment at m/z 15 from the parent molecular ion.

CH3COOH+ → CH3+ + COOH

Do i need the dot in front of the COOH ? Cause the solution didn't have it.

[Destiny]

I'm pretty sure you need the dot, but in front/at the back doesn't really matter. So long as it's there.

[soccerboi]

alright thanks