Samira's chem questions pls help

[Samira]

Q)Amino acid (zwitter ion)
Because amino acid are zwitter ion since the had both acid and base group. So if an acid for example glutamic acid was put in an alkaline solution will it donate 2 protons since it as 2 acidic group nd have 2 negative charge

Q2)Aspirin.....calculation
20g of salicylic acid was reacted with an excess of acetic anyhydrid. 18.5 g of acetylsalicylic acid ws isolated. Determine the yield for the reaction

3)how many water molecules would be required to hydrolyse a section of double stranded DNA containing a total of 20 nitrogenous bases into its constituent nucleotide???

[d0minicz]

Q1. yes

[dekoyl]

2) Calculate the amount of aspirin (acetylsalicylic acid) produced ignoring the "18.5g was isolated". You should find it's more than 18.5g.
Then,

[Toothpaste]

2. Percentage yield right?

Salicylic acid: ,
Acetic anhydride: ,
Acetylsalicylic acid: ,


+ +
(H+ from sulphuric acid ontop of the arrow thingy, forgot how to latex it in)

1:1:1:1

1.Salicylic acid: ,
okay ... you used 20g.
which means you used .1449mol

2.Acetic anhydride: ,
excess.

3.you got 18.5g ...or of Acetylsalicylic acid: .

theoretically you should've got of it because of the stoic. ratio.
(26.086957g)

Percentage yield: = 70.9% (you could even divide the mole, same thing)

[Samira]

THnx toothpick...it was in my trial exam but they had  no answer for it
Another question
A diluted vinegar sample was titrated with a starndised 0.1 M solution of sodium Hydroxide. By referring to the acid base indicator table in data booklet, name a suitable indicator for this titration,EXPLAIN your choice

[NE2000]

For Q3 you've got two strands each with 10 nucleotides each. To break up each strand you need 9 water molecules so in total you would need 18 water molecules I think.

For your question on vinegar.

This is a reaction between a weak acid and a strong base. At the equivalence point, both the weak acid and strong base would have been used up but the solution would still contain the week acid's conjugate base (the ethanoate ion) which being a base can accept protons from water and hence increase the pH of the solution. Hence at the equivalence point the pH of the solution would be > 7. The indicator required should hence change colour at a pH greater than 7. Phenolpthalein generally is the best choice for strong base + weak acid.

[Samira]

Thnx NE2000


i've got nother qs...
concentration on Cu ions is 1370 microgram/L
Its found the Cu ions came from the compound CuCl2. The mass of CuCl2 in mg in 1.0 ml sample of water is
A) 0.026
B)1.25
c)2.9
D)3.2

The answer is  2.9 but i keep getting 2.89*10^-3 .... i try to find the mass in 1 ml firs but the explanation for the answer straight away converts the mass to mole and find the mass CuCl2 without trying to find out in 1 ml becasue there is 1370 microgram in 1 L ... shouldn't we find out for i ml.......i'm confused

by the was its the 2nd qs from Tssm 09

[Samira]

anothe qs
A 100 mL solution of sodium hydroxide acid has a pH of 13. What volume of water, in mL,
must be added to it to change the pH to 11?
A. 2.0
B. 19
C. 99
D. 9900

cant seem to get the answer......... [frustrating]

[d0minicz]

is urs in terms of mg or grams ?

[Samira]

mg

[d0minicz]

NaOH ---> [H+] = 10^-ph .... so 10^-13 is conc.
we wanna find OH conc since we have NaOH
soo its [H+][OH-] = 10^-14      ---> [OH-] = 10^-14/10^-13
                                                               = 0.1M
pH of 11 --> do same thing
[H+] = 10^-11
[OH-] = 10^-3  ---> 0.001 M
Use C1V1= C2V2
100mL x 0.1M = 0.001 x V2
V2 = 10 000 mL
Soo we need 10 000 - 100mL = 9900 mL

[Samira]

thnx dominicz i dnno why i didnt see Tha NaOH.......thnx heaps

[Samira]

Another qs
The end point of redox titrtion can be determine using starch as an indicator. Starch will be blue in the presence of iodine. If you are performing this titration, EXPLAIN how you would know when to stop adding from the burette.

[Mao]

Deliver iodine from the burette. When the solution turns blue, end point is reached.

[Samira]

Another qs...which is from TSFX..
q6ii) A Particular geen consists of thirty nucleotides. How many molecules of water would be required to completely hydrolyse this gene.

does'nt this mean that there are 2 base pairs so 15 in each therefore we need 28 moleule of water to hydrolyse the base pairs den i'm lost bout the rest as in why is it times 30*2 nd if it is 60 wouldn't it be 60+28 not 29...........i'm confused