May 23, 2022, 07:33:25 PM
Forum Rules: Read This Before Posting

### Topic: Problem of the week - 17/12/2012  (Read 16901 times)

0 Members and 1 Guest are viewing this topic.

#### Borek

• Mr. pH
• Deity Member
• Posts: 27084
• Mole Snacks: +1759/-405
• Gender:
• I am known to be occasionally wrong.
##### Problem of the week - 17/12/2012
« on: December 17, 2012, 12:54:08 PM »
100 g of a mixture of two nitrates - A(NO3)2 and B(NO3)2 - was roasted, producing metal oxides and mixture of two gases - NO2 and O2. After passing the mixture through NaOH solution gas volume decreased 6 times, leaving 3620 mL of the gas (measured at STP, 0°C, 1 atm).

Determine A and B, knowing that one is an alkaline earth metal, and the other a transition metal.
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info

#### curiouscat

• Chemist
• Sr. Member
• Posts: 3005
• Mole Snacks: +121/-35
##### Re: Problem of the week - 17/12/2012
« Reply #1 on: December 18, 2012, 02:18:38 AM »
100 g of a mixture of two nitrates - A(NO3)2 and B(NO3)2 - was roasted, producing metal oxides and mixture of two gases - NO2 and O2. After passing the mixture through NaOH solution gas volume decreased 6 times, leaving 3620 mL of the gas (measured at STP, 0°C, 1 atm).

Determine A and B, knowing that one is an alkaline earth metal, and the other a transition metal.

To try and not give away the answer (in case I am right!  ):

ΔCAS-#(A,B) = ±428?

If 2% is acceptable error I get another pair that'll work too.

ΔCAS-#(A,B) = ±340?

Wonder if I'm making a blunder now.
« Last Edit: December 18, 2012, 02:55:42 AM by curiouscat »

#### Borek

• Mr. pH
• Deity Member
• Posts: 27084
• Mole Snacks: +1759/-405
• Gender:
• I am known to be occasionally wrong.
##### Re: Problem of the week - 17/12/2012
« Reply #2 on: December 20, 2012, 09:14:31 AM »
No other takes?

To be honest, using the answer I consider correct I am unable to get the differences of CAS numbers listed by curiouscat.
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info

#### curiouscat

• Chemist
• Sr. Member
• Posts: 3005
• Mole Snacks: +121/-35
##### Re: Problem of the week - 17/12/2012
« Reply #3 on: December 20, 2012, 12:35:55 PM »
No other takes?

To be honest, using the answer I consider correct I am unable to get the differences of CAS numbers listed by curiouscat.

Hmm...maybe I'm wrong then.

Just to confirm: My answer was Mn, Ba.

A little off, yet close was Co, Ra.

If wrong, I'll give it another shot.

#### Borek

• Mr. pH
• Deity Member
• Posts: 27084
• Mole Snacks: +1759/-405
• Gender:
• I am known to be occasionally wrong.
##### Re: Problem of the week - 17/12/2012
« Reply #4 on: December 20, 2012, 12:39:30 PM »
Mn & Ba is the correct answer. No idea how you used CAS numbers. Neither these for nitrates nor those for pure metals fit.
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info

#### curiouscat

• Chemist
• Sr. Member
• Posts: 3005
• Mole Snacks: +121/-35
##### Re: Problem of the week - 17/12/2012
« Reply #5 on: December 20, 2012, 12:49:09 PM »
Mn & Ba is the correct answer. No idea how you used CAS numbers. Neither these for nitrates nor those for pure metals fit.

Mn 7439-96-5
Ba  7440-39-3

7440393 - 7439965 = 428

Was I too cryptic? Am I making a blunder still?  Glad at least my underlying answer, sans antics, was right.

PS. Co, Ra is really close too. Have you checked? I can present my calcs. I got ~2% error in MW.

#### curiouscat

• Chemist
• Sr. Member
• Posts: 3005
• Mole Snacks: +121/-35
##### Re: Problem of the week - 17/12/2012
« Reply #6 on: December 21, 2012, 07:35:42 AM »
To justify the Co, Ra pair

3 Co(NO3)2  Co3O4 + 6 NO2 + O2

2 Ra(NO3)2  2 RaO + 4 NO2 + O2

The question is, does RaO exist? I'm not sure.

#### Borek

• Mr. pH
• Deity Member
• Posts: 27084
• Mole Snacks: +1759/-405
• Gender:
• I am known to be occasionally wrong.
##### Re: Problem of the week - 17/12/2012
« Reply #7 on: December 21, 2012, 08:11:35 AM »
Mn 7439-96-5
Ba  7440-39-3

7440393 - 7439965 = 428

I checked the first group of digits only for metals, even if I checked both variants (just first group and all digits) for nitrates
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info

#### curiouscat

• Chemist
• Sr. Member
• Posts: 3005
• Mole Snacks: +121/-35
##### Re: Problem of the week - 17/12/2012
« Reply #8 on: December 21, 2012, 08:24:11 AM »
I checked the first group of digits only for metals, even if I checked both variants (just first group and all digits) for nitrates

Next time I should just post the md5sum or rsa hash.

#### curiouscat

• Chemist
• Sr. Member
• Posts: 3005
• Mole Snacks: +121/-35
##### Re: Problem of the week - 17/12/2012
« Reply #9 on: December 23, 2012, 02:07:36 AM »
To justify the Co, Ra pair

3 Co(NO3)2  Co3O4 + 6 NO2 + O2

2 Ra(NO3)2  2 RaO + 4 NO2 + O2

The question is, does RaO exist? I'm not sure.

Haven't got any comments on the Co,Ra answer so I'll post some more details:

0.1616 gmol O2 and 0.808 gmol NO2 were liberated. 0.2424 gmol Co(NO3)2 and 0.1616 gmol Ra(NO3)2.

Gives 344.4 as MW of Ra(NO3)2. Whereas real MW is 350.  Less than 2% error.

@Borek: What do you think?

#### Borek

• Mr. pH
• Deity Member
• Posts: 27084
• Mole Snacks: +1759/-405
• Gender:
• I am known to be occasionally wrong.
##### Re: Problem of the week - 17/12/2012
« Reply #10 on: December 23, 2012, 05:15:43 AM »
Nothing unusual in this type of the question that within higher tolerances other answers fit the data.

Difficult to comment on the chemistry. It does sound reasonable, although as you signaled RaO seems to be elusive. It could be interesting to see thermogravimetric curves, somehow I doubt one for Ra(NO3)2 exists.
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info

#### curiouscat

• Chemist
• Sr. Member
• Posts: 3005
• Mole Snacks: +121/-35
##### Re: Problem of the week - 17/12/2012
« Reply #11 on: December 28, 2012, 01:42:38 AM »
Nothing unusual in this type of the question that within higher tolerances other answers fit the data.

Difficult to comment on the chemistry. It does sound reasonable, although as you signaled RaO seems to be elusive. It could be interesting to see thermogravimetric curves, somehow I doubt one for Ra(NO3)2 exists.

This was a very informative paper about nitrate decomposition:

http://www.nist.gov/data/PDFfiles/jpcrd11.pdf

Very thorough. If anyone finds it useful.

#### Yggdrasil

• Retired Staff
• Sr. Member
• Posts: 3216
• Mole Snacks: +484/-21
• Gender:
• Physical Biochemist
##### Re: Problem of the week - 17/12/2012
« Reply #12 on: December 28, 2012, 12:16:31 PM »
It's worth noting that at low temperatures NO2 dimerizes to form N2O4, so the pressure would decrease only 3.5 fold after the NaOH solution.

#### curiouscat

• Chemist
• Sr. Member
• Posts: 3005
• Mole Snacks: +121/-35
##### Re: Problem of the week - 17/12/2012
« Reply #13 on: December 28, 2012, 01:01:30 PM »
It's worth noting that at low temperatures NO2 dimerizes to form N2O4, so the pressure would decrease only 3.5 fold after the NaOH solution.

Ha! Never thought of that!