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Topic: Questions on Acid salts, Iron(III)sulphate, test for nitrate, Alum.Oxide...  (Read 10067 times)

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gwx

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Hi, I have some questions about chemistry and hope I can get as much info as possible. I couldn't find information on google, nor in my 2 textbooks and I even went to the library and searched another textbook.. to no avail. Pleaes please help by answering any thing that you know.. its for my test tommorow!

1. Question of acid salts..
My textbooks say that sodium hydrogencarbonate is an acid salt... Does it mean that when you dissolve it in water, the pH is less than 7?

My chemistry teacher said ( some time ago ) that when a weak acid like carbonic acid, reacts with a strong alkali like sodium hydroxide, then the salt formed is alkaline... A chemistry website agrees with him.. "Salts made of the anion of a strong acid and the cation of a strong base will be neutral salts, that is, the water solution with this salt will have a pH of seven. (example - sodium chloride)

Salts made of the anion of a strong acid and the cation of a weak base will be acid salts, that is, the water solution with this salt will have a pH of less than seven. (example - ammonium chloride)

Salts made of the anion of a weak acid and a strong base will be an alkali salt. The pH of the solution will be over seven. (example - sodium bicarbonate)

It can be a bit more difficult to tell the pH of a salt solution if the salt is made of the anion of a weak acid and the cation of a weak base. Usually, the main determining factor is whether the weak acid is weaker than the weak base, but that is not always the case. For the purpose of the problems in the review section, you may say that the pH of a 'weak-weak' salt is indeterminate." from http://www.chemtutor.com/acid.htm#phst

However, my textbook says sodium hydrogencarbonate is ACID salt!! Is it that even tho its an acid salt, when dissolved in water it is alkaline??

2. Iron (III) Sulphate

I have done a titration with iron(III) sulphate and  iron(II) sulphate ( which is colourless if I remember correctly? is it?? please confirm with me.... )

Ok I did this some time back so cannot remember clearly

But one thing I do remember clearly is that there is a kind of greenish tint to the solution when KMnO4 and FeSO4 is mixed.... before the endpoint ( which is pale pink ). What is this greenish tint about? The chemical equation on the right-hand-side has potassium sulphate, manganese sulphate, iron sulphate and water... I suspect the greenish tint is  a combination of all these chemicals??

I would also like to clarify certain things...

What is the colour of iron(II)sulphate crystals?? ( i read from internet... its blue-green, true??? )
What is the colour of iron (III) sulphate solution and its crystals( I don't know and cant find any info... )

3. Test for nitrate
The textbook test for nitrate invovles

1. add dilute sodium hydroxide
2. add a bit of aluminium powder
3. if nitrate is present, ammonia gas is produced..

How do these chemicals react??? Does sodium hydroxide react with the nitrate?? ( I doubt so.. ) or does aluminium powder react with nitrate ( I dont know! )... Please teach me how this works cause in the textbook and internet theres nothing on it!!! Chemical equations will be helpful too thanks!

4. Aluminium Oxide

The layer of Al2O3 outside aluminium... it readily dissolves when Mercury (II) Chloride is rubbed aganst it... What's the chemistry behind this??? Cant be displacement :(

5. Hydrochloric acid and potassium permanganate...

Do they react??

Again, all these questions I ask here because there's no information on google so its a last resort... Please help... my chem common test is tommorow! ( eh.. its no really a big exam but i want to get better grades this time! )


 ;)
« Last Edit: April 05, 2005, 12:13:49 PM by geodome »

Offline hmx9123

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Question 1:

Sodium hydrogen carbonate is sodium bicarbonate.  You already answered your question.  You state plainly what sodium bicarbonate is.  What did you determine the formula of each to be?

Question 2:

The reaction between iron (III) sulfate and potassium permanganate actually has pictures in my gen chem book.  The color of iron(III) sulfate can be found easily in the CRC handbook of chemistry and physics.  You are right to look at the colors of both iron(III) and iron(II) sulfate, both in solution.  That will help you figure out the color change.

Question 3:

This is actually quite common to read about on the internet, provided you're reading in the pyrotechnic section... like rec.pyrotechnics.

http://groups-beta.google.com/group/rec.pyrotechnics/browse_thread/thread/ddd09ef238690c9d/505748fedc45bc04?q=ammonia+aluminum+reaction&rnum=6#505748fedc45bc04

Question 4:

I believe that the mercury amalgamates the aluminum (forms an alloy with; i.e., dissolves the Al).  It probably is able to interstitally attack the aluminum.  This is not a trivial question.

Question 5:

It should for permanganic acid, which, IIRC, is an acid that is unstable in pure form, like chloric acid.  I'm sure AWK is going to correct me on all of this. :)

Best of luck with things.  BTW, if you need help, try asking things more than a day in advance.  Most of these questions would never be asked on a general chemistry exam anyway.
« Last Edit: April 05, 2005, 06:04:47 AM by hmx9123 »

gwx

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Thanks for replying!

I'm taking the GCE O levels papers at the end of the year and for this particular test of mine, I have done basically most of the preparations but I'm usually just very curious about why and how... Those questions I have I don't think have much likelihood of coming out... But I need to know the clours of Iron(iii) sulphate?? I don't have the CRC book :( ( btw, I live in singapore, so we use different textbks.... )


Offline xiankai

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Q1: sodium bicarbonate is baking soda, so its an basic salt?

Q2: iron(II) sulphate is green in colour, iron(III) sulphate is brown

i dont know how u got colourless for your titration...

but in your equation with end products with potassium sulphate, maganese sulphate (think it shld be oxide tho) and iron(II) sulphate, the first two products are colourless but the last is green, hence the greenish tint. it is NOT a result of the products reacting together. they dont.

the rest well im also looking for answers :P

-edit- u're from singapore too? i am! and im taking the o' lvls too :P
« Last Edit: April 05, 2005, 09:23:13 AM by xiankai »
one learns best by teaching

gwx

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Ummm sorry!

It was a typo.... I meant Iron sulphate and KMnO4!! ( for titration)

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