Chem Questions

[horizon]

Q1) How can monosaccharides be oxidised to produce smaller units? Aren't they already the smallest unit themselves?

Q2) It says that as the temperature increases, Rf values will increase.  However I don't get this because if won't the ratio still remain the same no matter how far the solvent front and its components are able to move?

[Nobby]

From my knowledge:
1. Oxidation of a monosaccharide will produce a carboxylic acid.
2. I would've though lower R_f values, as higher temperature means more collisions, meaning more opportunities for the molecules to adsorb to the stationary phase. Hence, longer time adsorbed to the stationary phase, less time moving with the mobile phase, and lower R_f.

[horizon]

Thanks for your reply.  I'll probably have to double check the accuracy of the Rf value statement.

Q3) What does it mean to "oxidise" a fat?

[charmanderp]

I'm think an increased temperature might reduce tension between surfaces, allowing the component to move further up the stationary phase. I'm not sure exactly why but in HPLC an increased temperature decreases retention time, which is synonymous with increasing Rf values.

Oxidising a fat refers to the procedure of hydrolysis where a complex fat molecule is split into its more simple component molecules ie. three fatty acids and glycerol.

[illuminati]

Guys oxidation could just mean combustion.
Thats for both for "oxidation of a fat" as well as "oxidation of a sugar"

As for the increased temperature, i would ask for what the stationary and mobile phase are...

[thushan]

Not entirely sure myself, but I would think that it could occur either way - like increasing temperature may increase or decrease Rf depending on the components and the stationary and mobile phases. It could increase Rf if it turns out the compound dissolves better into the mobile phase at higher temperature, and that it could be the overriding factor - that would be in some cases only. Point is, temperature I think will change Rf yes, but in either direction depending on the compounds involved.

[horizon]

Thanks for your explanations!

Ok this question- is the answer wrong???

Q4) The pH of a 10^-8 M NaOH solution at 25 degrees Celsius is closest to:
A. 6
B. 7
C. 8
D. 10

The answer says B- 7! (thought it was 6...is this a trick question?)

[charmanderp]

You'd probably be safe saying that it would definitely not be six, seeing as NaOH is a base, and hence will likely have a pH of 7 or above. The pOH of base would indeed be 8, but due to the heavy dilution of water the pOH of the solution would be closer to 7, and hence the pH itself would be circa 7.

[yellowsone31]

Can HPLC be used for hydrocarbons?

[|ll|lll|]

Thanks for your explanations!

Ok this question- is the answer wrong???

Q4) The pH of a 10^-8 M NaOH solution at 25 degrees Celsius is closest to:
A. 6
B. 7
C. 8
D. 10

The answer says B- 7! (thought it was 6...is this a trick question?)

The answer is B. It is virtually impossible for the solution to be acidic given that there are Na+ and OH- ions in the solution.
In this case, the actual concentration of OH- ions is actually 10^-8 + 10^-7 (from H2O, you get 10^-7M of H+ and 10^-7M of OH-) = 0.00000011 = 1.1 x 10^-7
If you take the -log_10 [1.1 x 10^-7] of that, you'll get a pOH of 6.9586 ~ 7. Thus, pH = 14-7=7

Can HPLC be used for hydrocarbons?

Yes, when it is not volatile

[ggxoxo]

Can HPLC be used for hydrocarbons?

If the hydrocarbon has M>300 then I don't see why not.

Generally, M<300- GC
M>300- HPLC

[horizon]

Q5) 6.54 g of zinc metal is completely dissolved in 250 mL of concentrated hydrochloric acid to give gaseous hydrogen according to the equation Zn (s) + 2 HCl (aq) -->  ZnCl2 (aq) + H2 (g)

The concentration of zinc ions in the solution after all the zinc has reacted would be
a.   0.10 M
b.   0.20 M
c.   0.40 M
d.   0.025 M

Answer is B.... (?)

[Mao]

Q5) 6.54 g of zinc metal is completely dissolved in 250 mL of concentrated hydrochloric acid to give gaseous hydrogen according to the equation Zn (s) + 2 HCl (aq) -->  ZnCl2 (aq) + H2 (g)

The concentration of zinc ions in the solution after all the zinc has reacted would be
a.   0.10 M
b.   0.20 M
c.   0.40 M
d.   0.025 M

Answer is B.... (?)

The thoughts going through the guy who wrote the questions:

...I'll use 500mL of solution. Oh wait, a 500mL solution is like, a whole half a litre man.. That's going to be one giant piece of glassware. I better make this smaller... 250mL should be good. Oh yeah, I should update the answers too... Hey look! A distraction!

<2 months later />

(correct answer is C)

[thushan]

GOLD!

[horizon]

A6)
The mass of ammonium nitrate, NH4OH, of 80% purity, which contains 1.4 g of nitrogen is approximately
A.   1.4 g
B.   2.5 g
C.   3.2 g
D.   5.0 g

Answer: D...