Rishi's Chemistry Thread

[Blondie21]

A student titrates sulfuric acid against a 0.200 M sodium hydroxide solution. 20.00 ml aliquots of sodium hydroxide are used
How do I know if the acid or the base is in the burette?

think about it Rishi.. If there are 20mL ALIQUOTS of NaOH hmmmm where does this go? In the burette or in the conical flask?

[Rishi97]

A 20mL aliquot of sodium hydroxide solution, NaOH, is added to a flask from a stock solution and is titrated against 0.66M sulfuric acid. The average titre is 16 mL

1) Another 20mL aliquot of NaOH is taken from the same stock solution and is diluted to 100mL. The volume of 0.66M sulfuric acid required in the titration this time, should be, in mL

Thanks

[Blondie21]

A 20mL aliquot of sodium hydroxide solution, NaOH, is added to a flask from a stock solution and is titrated against 0.66M sulfuric acid. The average titre is 16 mL

1) Another 20mL aliquot of NaOH is taken from the same stock solution and is diluted to 100mL. The volume of 0.66M sulfuric acid required in the titration this time, should be, in mL

Thanks

Remember this: Diluting something will NOT change the amount of mol. Think about it like this... If I had 3 balls in a container of 50mL of water and I added another 100mL, would this change the amount of balls? No. Same with molecules. Keep this in mind.

n(NaOH) = 0.011mol (For the reaction with the 20mL aliquot)
Once this is diluted, the mol remains at the value. However, the concentration changes. It is now:
c(NaOH) = 0.11M

We know the concetration of sulphuric acid remains at .66M so
c1V1=c2V2
(0.11)x(0.1) = (.66)V2
V= 0.016L = 16mL

Orrrrr you could have skipped all of those calculations as the mol of H2SO4 remains the same. Because the mol and concentration is the same, the volume must be the same too!

[Rishi97]

Is the following statement correct regarding electrochemical series?

The more negative the E⁰ value is, the more readily oxidation occurs and the stronger the reducing agent

Thanks

[lzxnl]

Is the following statement correct regarding electrochemical series?

The more negative the E⁰ value is, the more readily oxidation occurs and the stronger the reducing agent

Thanks

Yes

[Rishi97]

List two factors that need to be considered when selecting an appropriate substance for use in a salt bridge

Thanks

[Rod]

List two factors that need to be considered when selecting an appropriate substance for use in a salt bridge

Thanks

Some off the top of my head;

- Whether it is soluble or insoluble
- Salt chosen cannot react with cell contents
- Whether it provides the appropriate ions to balance charges generated in each half cell

[Yacoubb]

List two factors that need to be considered when selecting an appropriate substance for use in a salt bridge

Thanks

This may be one reason, but really it covers everything. The salt being used should contain soluble ions (this is so that no precipitate forms in the half-cells, which would disrupt the experiment).

[Phenomenol]

Some off the top of my head;

- Whether it is soluble or insoluble
- Salt chosen cannot react with cell contents
- Whether it provides the appropriate ions to balance charges generated in each half cell

I'm not sure if the third point is necessary.
There was a terrific question on this topic in the VCAA 2013 exam.

[lucas.vang]

Hi everyone, I have a question regarding the products of electrolysis

When it is something like potassium nitrate?
How would I predict it when nitrate isnt on the electrochemical series?
and what about the others like sulphate and others?

thanks in advance

[lzxnl]

Hi everyone, I have a question regarding the products of electrolysis

When it is something like potassium nitrate?
How would I predict it when nitrate isnt on the electrochemical series?
and what about the others like sulphate and others?

thanks in advance

There is always water to electrolyse. Never forget that.

VCAA won't ask you about the others. You can reduce sulfate and nitrate but VCAA won't ask you for any reaction that's not in the data book unless they give you the reaction.

[Rishi97]

Which of the following compounds contain the highest percentage by mass of oxygen?
a) Propyl propanoate
b) 1-propanol
c) Propanoic acid
d) 1- aminopropane

I've ruled put b and d but I can't decide between a and c

Thanks guys

[lzxnl]

Which of the following compounds contain the highest percentage by mass of oxygen?
a) Propyl propanoate
b) 1-propanol
c) Propanoic acid
d) 1- aminopropane

I've ruled put b and d but I can't decide between a and c

Thanks guys

Propanoic acid is C3H6O2
Propyl propanate is the same as the above, except you're substituting a hydrogen for a CH3CH2CH2 group
So it has the same number of oxygens with a higher molecular mass => lower percentage by mass of oxygen

[Rishi97]

The pH of water is 7.0 at 25 degrees and it is 7.5 at 0 degrees. This means that:
A. water is not neutral at 0 degrees
B. the self-ionization of water is an endothermic process
C. water becomes more alkaline as the temperature drops
D. the percentage of ionization increases as the temperature drops

I always get these sorts of questions wrong. Is there like a basic theory that I'm missing?

[nhmn0301]

The pH of water is 7.0 at 25 degrees and it is 7.5 at 0 degrees. This means that:
A. water is not neutral at 0 degrees
B. the self-ionization of water is an endothermic process
C. water becomes more alkaline as the temperature drops
D. the percentage of ionization increases as the temperature drops

I always get these sorts of questions wrong. Is there like a basic theory that I'm missing?

A neutral solution (water in this case) has [OH-] = [H+]. So if at 25C, you have pH = 7, hence, [H+]=[OH-]= 10^(-7) => Kw= 10^(-14). At 0 C, pH= 7.5, hence, [H+]=[OH-]= 10^(-7.5) => Kw= 10^(-15). This means that as temperature decrease, Kw decreases, indicating that the self-ionization of water is endothermic. So I think the answer is B.