Homework questions thread

[charmanderp]

A peak at 29 would be produced by a CH3CH2+ fragment. Not sure about the others, though.

[Mr. Study]

^ Regarding that, I remember my teacher saying that peaks can be produced by any fragment, if it was 'cut off' from the main compound.

Like the ester I posted previously could be produced by the CH3CH2+, like you said, but due to what my teacher said, I thought that a fragment could also exist if we 'cut off' the 2 Hydrogens and the C-C bond, to get the CH3C+.

That probably sounds ridiculous but that was the impression I got from my teacher. >.>

[charmanderp]

But a CH3C+ peak (which I'm not even sure is possible) would be manifest at 27 m/e, not 29!

[Mr. Study]

Ahh! My bad, I checked out my sheet and it was CH3CH2+.

NVM, Thanks though charmanderp. Updated my original post to change the contention of my question now!

[Somye]

I think a CH3C+ would be extremely unstable, and so even if it did show up, the peak height would be almost neglible

[soccerboi]

Could someone help me with this one?
Citric acid, the predominant acid in lemon juice, is a weak, triprotic acid. A student titrated 25.00ml samples of lemon juice with 0.550M NaOH (aq). The average titre was 29.5 ml. The molar mass of citric acid is 192 g mol^-1. What was the concentration, in g L^-1, of the citric acid in the lemon juice?
Answer is 41.4 g L^-1

[charmanderp]

Since it's a triprotic acid and NaOH accepts one hydrogen atom to form H2O, we need three mole of NaOH for every mole of citric acid.

We have 0.016225mol of NaOH, so we take one third of that to find the amount of citric acid in the 25.00ml aliquot. Which equates to 0.005408333333mol. Then we divide that by the volume of the aliquot to find concentration, which gives us 0.216333333M

Since M is mol/L, we simply need to multiply concentration by molar mass to find concentration in g/L, so therefore 0.216333333*192=41.5 g/L, and I'd assume my rounding has caused that answer to differ ever so slightly from yours.

[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

[soccerboi]

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

[charmanderp]

If an acid is monobasic, like HCl, it has one hydrogen atom to donate to a base in an acid-base reaction. Therefore, when NaOH and HCl are reacted, the mole ratio is 1:1, as NaOH can accept one hydrogen atom to form H2O and NaCl.

If an acid is dibasic, like H2SO4, it can donate two hydrogen atoms. Since NaOH can only accept one per molecule, two NaOH molecules are required, hence the mole ratio is 2(NaOH):1(H2SO4)

If the acid is tribasic, or triprotic, like H3PO4, it donates three hydrogen atoms. Therefore we would need three molecules of NaOH for every molecule of H2SO4, giving us a mole ratio of 3(NaOH):1(H3PO4).

Most of the time you won't need to know this, all you'll have to do is write a balanced equation and it will make itself obvious.

[ecvkcuf]

Find the molar mass of a protein if each molecule contains  4 iron atoms and the iron makes up 0.33% of the the mass.. how do I find the molar mass of it?

[charmanderp]

Do you know what the answer is?

[soccerboi]

Find the molar mass of a protein if each molecule contains  4 iron atoms and the iron makes up 0.33% of the the mass.. how do I find the molar mass of it?

4X 55.8 = 223.2 g of iron in the molecule.

(223.2/x)X100=0.33
rearrange and solve for x. u get 67 636g mol^-1
The answer seems very large though...
Hope that helps

[soccerboi]

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?

[soccerboi]

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?