Exam Suggested Solutions

[Mao]

VCE Chemistry Unit 3 Exam

Multiple Choice Questions

Question 1
B, 14.58mL

Question 2
A, 5.13E9 molecules

Question 3
Xylose is a compound that has five carbon atoms in each molecule and contains 40% carbon by mass. What is the molar mass of xylose?

C, Mr=150

Question 4
D, Phenolpthalein red to colourless [base to acid]

Question 5
D, Mr(Glu) > Mr(Val), and NMR is different
though the environments of the Z in Val and Leu are the similar, it should be noted that at least the integration trace would be different.

Question 6
Aspirin is a compound widely used as a painkiller and to relieve the symptoms of fever. It can be produced by means of a reaction in which salicylic acid is one of the reagents.
The structures of aspirin and salicylic acid are shown below.
which of the following statements about aspirin is not correct?
A. Aspirin may be prepared by reaction between salicylic acid and CH3OH
B. Aspirin contains both an ester and a carboxylic acid functional group
C. Aspirin can undergo an acid-base reaction with NaHCO3
D. Aspirin may be prepared by reaction between salicylic acid and CH3COOH

A, aspirin SA + methanol

Question 7
C, alkene and amide

Question 8
A, DNA

Question 9
D, 150 molecules of G

Question 10
B:
piece 2 will be most easily separated [less GC pairs]
strand W will be heavier, as both G and A are pyrimidine [two rings]

Question 11
Gel electrophoresis is a technique that can be used to separate DNA fragments in forensic chemistry. The gel resulting from such a separation experiment carried out at pH 7 is shown below

Code: [Select]

   X                    Y
:  |    |                |
:  |    |                |
:  |    |                |
Which of the following statements about the experiment is not correct?
A. Fragment X has a higher molecular mass than fragment Y
B. Fragment Y moves through the gel at a faster rate than fragment X
C. The negative terminal of the power supply is connected to the end A of the gel
D. Under the conditions of this experiment, the DNA grament are positivelyu charged

D. Gel electrophoresis does not have positively charged DNA fragments in pH 7

Question 12
A, identity is determined by retention time

Question 13
D, relative amount is determined by area under the peak

Question 14
C, AAS would not provide information about a biomolecule

Question 15
D, magnetic field -> nucleic energy

Question 16
How many structural isomers, each containing a double bond, have the molecular formula C5H10?

C, 5 isomers:

Code: [Select]

c-c-c-c=c

c-c-c=c-c

c-c-c=c
  |
  c

c-c-c=c
    |
    c

c-c=c-c
  |
  c

Question 17
B, it has alkene (react with Br₂) as well as COOH (react with sodium carbonate)

Question 18
C, condensation of 2235. the fact that water is lost during polymerisation has to be factored in.

Question 19
B, linoic. there are two alkene groups.

Question 20
A, Y is more volatile than Z

B and C are both false, and D: "Z has stronger covalent bonds" is not necessarily true


Short Answer Questions

Question 1
The percentage purity of powdered, impure magnesium sulfate, MgSO4, can be determined by gravimetric analysis. Shown below is the method used in one such analysis.

Method:
32.50 g of the impure magnesium sulfate is dissolved in water and the solution is made up to 500.0 mL in a volumetric flask.
Different volumes of 0.100 M BaCl2(aq) are added to six separate 20.00 mL samples to this solution. This precipitates the sulfate ions as barium sulfate. (equation cbf'd)
The precipitate from each sample is filtered, rinsed with de-ionised water and then dried to constant mass.

Results
Mass of impure magnesium sulfate = 32.50 g
Volume of volumetric flast = 500.0 mL
Volume of magnesium sulfate solution in each sample = 20.00mL

sample	1	2	3	4	5	6
Volume of BaCl2 (mL)	30.0	60.0	90.0	120.0	150.0	180.0
mass of BaSO4 (g)	0.704	1.41	2.00	2.00	2.00	2.00

a. Why is it necessary to rinse the precipitate with deionised water before drying? (1 mark)
-> to ensure there are no other ions present in the filtrate

b. Explain why the amount of BaSO4(s) precipitated remains constant for the last 4 samples tested even though more BaCl2 was added (1 mark)
BaCl2 is in excess

c
    i. calculate the amount of SO₄^2-(aq) in the 500mL volumetric flask (2 marks)
    0.214 mol
   
    ii. calculate the percentage by mass of magnesium sulfate in the powder (2 marks)
    79.4%

Six further 20.00mL samples of the manesium sulfate solutions are analysed. However, the concentration of the barium chloride added to those six samples is 0.200M
d (2 marks)
the graph:
30mL = 1.408 g of sample
line at ~45mL = 2g
all dots afterwards are at 2.00g

Question 2
One method of analusing the manganese content of steel is to dissolve the steel in nitric acid, producing a solution of manganese II ions

the Mn2+ ions are then treated with an excess of acidified solution of periodate ions, I)₄^-. The products of this reaction are iodate ions, IO3-, and the deeply purple coloured permanganate ions, MnO4-

The concetration of MnO4- is then determined by UV vis.
a
    i Calculate the oxidation number of iodine in IO_4^- (1 mark)
    +7

    ii. Give a half equation for the conversion of IO_{4^- to IO3^-} (1 mark)
   

    iii. is the IO_{4^- ion acting as oxidant or reductant?} (1 mark)
    oxidant, it gains electrons
An experiment was carried out to determine the percentage of manganese in a particular sample of steel by the above method.

A 13.936 g sample of steel was dissolved in acid and the manganese was converted to MnO4-
The solution containing the MnO4- ions was filtered and made up to a volume of 1.00L
25.00mL of this solution was then futher diluted to a 100.0 mL in a volumetric flask. the absorbance, at 525 nm, of this solution was 0.70
[and a graph of standards giving 35mg/L]
b
    i. /what is the concentration of MnO₄^- in the diluted 100mL (1 mark)
    from graph, 0.70 => 35 mg/L
   
    ii. Calculate the mass of manganese in the steel sample (2 marks)
    64.6 mg

    iii. Calculate the percentage by mass of manganese in the steel sample (1 mark)
    0.464%


Question 3

a. write the reagent (1 mark)
CH₃COOH (or draw the structural formula)

b
    i. draw the component with Rf=0.4 when solvent from =5 (1 mark)
    line at 2

    ii. 4 aminophenol adsorbs less strongly to the stationary phase (1 mark)
    Rf would be greater because:
    less adsorption onto stationary -> more time in mobile -> more movement -> higher displacement -> higher Rf

c (1 mark)

this question is a bit nasty:


there will be a bit of leeway on this as it is interpreting the number from the graph. if you've wrote 1.6 for absorbance of first, the answer 9.06

Question 4

Compount X at 120C, 115kPa, 0.376g occupies 124mL
a. Calculate the molar mass of X (2 marks)
86.1 g/mol

Compound Y is C₄H₁₀O
b
    i. draw the structural formulas (4 marks)
    (i cbf doing it all out, but here's the general structures)
   

Code: [Select]

C-C-C-C-O

    O
    |
C-C-C-C

C-C-C-O
  |
  C

  O
  |
C-C-C
  |
  C
   
    Y has 3 lines in C NMR, and react with Cr₂O₇^-
    ii (2 marks)
    Evidence from NMR: 3 carbon environments
    Evidence from second reaction: 1-hydroxy (-OH is at the end of the backbone)

    iii (2 marks)
    2-methylpropan-1-ol

Compound Z has the molecular formula C5H10O, and shows a strong band in the infrared spectrum at about 1700
c
    i. (2 marks)
    IR ->  C=O group
    NMR -> 2 H enviroments. CH3-CH2-

    ii (1 mark)
    hydrogen are not drawn
   

Code: [Select]

    O
    ||
c-c-c-c-c
bahh! and we thought ketone wasnt on the course

Question 5
a
    i. Draw the structure of serine in pH 2 (1 mark)
    cbf.. H₃N⁺-...

    ii. Name the functional group on the side chain (1 mark)
    hydroxy/hydroxyl, NOT alcohol

b. write the molecular formula for phenyalanine (1 mark)
C9H11NO2

c
    i. draw the structure of dipeptide (1 mark)
    bah--hab

    ii circle the amide/peptide (1 mark)
    see above

d. sketch covalent link of the interaction between cysteine (1 mark)
-S-S-
disulfide


Question 6
a. Give a systematic name for each of the following compounds
    i (2 marks)
    ethylamine

    ii (2 marks)
   

Code: [Select]

    Cl    c
    |     |
c-c-c-c-c-c
          |
          c
    5-chloro,2-methylheptane

C3H7Cl + X -> K, K + Cr2O7^2-, H+ -> C3H6O2
                                                                            + H2SO4 -> methyl propanoate
                                                                   L
b
    i. draw methyl propanoate (1 mark)
    bah~O-(O=C)~bah

    ii. draw K and L (2 marks)
    K is 1-propanol (c-c-c-o)
    L is methanol (c-o)

    iii. write reagent X (1 mark)
    NaOH

Question 7

a. Write a chemical equation for the combustion of ethnaol in excess oxygen (2 marks)


b. Give chemical reasons why ethanol can mix with both water and petrol (2 marks)
-> OH functional group => miscible in water
-> hydrocarbon body => miscible in petrol

c.
    i. Write a chemical equation for the fermentation of glucose to produce ethanol (1 mark)
   

    ii. Explain why ethanol produced by fermentation is refferred to as a "biochemical fuel" (1 mark)
    it is a biological process in yeast, i.e. conversion of glucose [biological] into ethanol [fuel]

d. What fragment would have been lost from the molecular ion to accound for m/z of 45? (1 mark)
H in OH

Question 8

0.415g of pure acid, H2X, is added to exactly 100mL of 0.105M NaOH
A reaction occurs (etc)

The NAOH is in excess. This excess NaOH requires 25.12 mL of 0.197M Hcl for neutralisation

i. calculate the amount in mol of NaOH added initially (1 mark)
0.0105 mol

ii. calculate the amount in mol of NaOH that reacts with the acid H2X (2 marks)
5.53E-3 mol

iii. calculate the molar mass of H2X (2 marks)
150 g/mol

(i lost one mark here forgot that it was diprotic )

[onlyfknhuman]

i found it pretty hard, im very weak at stoichemtry shortanswers seemed to revovled around it -_-'.

all the study guides/sample exam were so differnt to the real thing wat the fook!

[Marc]

I think i did really well, the only questions i think i stuffed up were the drawing of the C5H10O molecule and the manganese amount in the steel alloy.. But overall a good paper =)

[Rosie]

Who agrees with me that there just wasn't enough time. This is what stuffed me up. Left heaps of questions out when lady called out that there was 5mins to go.

[monicak]

I *think* most of my solutions were same as or similar to your answers. I had a bit of trouble with question 1. Not sure why, as it's simple gravimetric/calculations, but I double checked it and still couldn't find what I did wrong. Would you be able to put up the actual question and working for question 1c parts i) and ii)? I really don't know how I stuffed it up 

[Collin Li]

If anyone has any specific queries about a question, PM me or MSN me with a link to the exact post of your inquiry.

(Right click the link of your post "Re: Exam Suggested Solutions *under development*" and copy target location. Example: http://vcenotes.com/forum/index.php/topic,3799.msg44838.html#msg44838)

The "#msg..." at the end of the link should be there, which directs my browser to your exact post, so I don't have to go looking for it.

If I know anything about how Exam discussion threads go, they move like wildfire, so I'm not going to be checking very actively. However, if you have a specific concern, copy me (or explain to me) the question, and I will answer.

[monicak]

Who agrees with me that there just wasn't enough time. This is what stuffed me up. Left heaps of questions out when lady called out that there was 5mins to go.

I managed to finish it with a couple of minutes left over, didn't have time to check all my solutions.. But I do agree that it was quite long. The questions weren't particularly hard, but the Multiple Choice seemed to take me a long time... Quite a few people from my school didn't finish the last few pages..

[higbvuyb]

Could someone post the question text of multiple choice questions 11 and 16?

[bucket]

Hah so far I've gotten the majority right , or at least my answers were consistent with mao's,

[vce01]

Could someone post the question text of multiple choice questions 11 and 16?

and MC 6 as well, along with the options please!

[Collin Li]

Mao, for Question 16, did the question have a requirement on those isomers? In particular, I'm looking for "straight-chain" isomers, because cyclopentane is also another structural isomer.

Of course, there are also some geometric isomers, but they aren't structural isomers, nor are they on the course. A clever forum member just PM'd me about this.

EDIT: Spotted the restriction: "containing a double bond"

[username]

What do you guys think will be the cutoff for a B this year?

[Tea.bag]

can u please put up question 3 for the multiple choices

[ausyid]

Mao, for Question 16, did the question have a requirement on those isomers? In particular, I'm looking for "straight-chain" isomers, because cyclopentane is also another structural isomer.

Of course, there are also some geometric isomers, but they aren't structural isomers, nor are they on the course. A clever forum member just PM'd me about this.

I was about to say, I answered D for that question. There is a sixth isomer in which one Carbon has four carbons coming off it surely?

[Collin Li]

I was about to say, I answered D for that question. There is a sixth isomer in which one Carbon has four carbons coming off it surely?

Sorry, the question required the isomers to have "a double bond." The sixth one I was thinking of was cyclopentane, which doesn't have a double bond.

There is no carbon that has four methyl groups coming off it, because then it would be C5H12. A double bond cannot be placed anywhere, as it would force the central carbon to have 5 bonds.