Can't Judge a Powder B

[starpug]

Oops. My bad. I thought it had two sulfur molecules. I hate my partner!!
A weak acid? We tested for a pH of 1 or 2.

It's a common mistake

Also the term "weak acid" doesn't mean it can't have a ph of 1 or 2. pH is a measure of H+ ion concentration, so a saturated solution of say citric acid (what makes lemons so sour) can have a pH as low as or lower then a dilute solution of HCl. What makes HCl and other strong acids strong is that they completely dissociate in water so if you were to add the same number of moles of HCl and citric acid to water, the HCl would have the higher pH. The HCl would donate more H+ ions to the solution then the citric acid (making more Hydronium Ions, but the difference between H+ and H30+ is irrelvant to what I'm saying.) which makes the pH lower (they're inversely proportional since pH is the -log[H+]). Even among weak acids, some are stronger then others because some dissociate better then others. Also, pH paper has a history of being unreliable, so it's possible that was a faulty reading.

Ferric Acid's formula H2FeO4, I can't seem to find it's dissociation constant so I don't know how strong of a weak acid it is.

I have always thought a weak acid meant an acid with a pH close to neutral. When my partner and I tested the pH of Ferric Acid (or whatever it is) we got a pH of 4. Would that be considered a weak acid?

Let me say this, pH does not tell you whether it is strong or weak acid, unless you know the concentration of the solute in the solution as in that case the H+ concentration you get from the pH (if the pH was 4 the H+ concentration would be .0001 mol/L) matches the total concentration of the solute in the solution you have a strong acid. If it were less then the total concentration it would be weak.

I don't think they gave you enough data to do that, so it's much easier to memorize the strong acids
HCl - Hydrochloric
HI - Hydroiodic
HBr - Hydrobromic
H2SO4 - Sulfuric
HClO4 - Perchloric
HClO3 - Chloric
HNO3 - Nitric

Anything else is a weak acid

[frogzorz]

Ahh, I see. Thanks
Our pH paper is pHydrion but it might be a bit old.

[frogzorz]

Yeah, the question was like "Besides a rotten egg odor, what did you observe when ferric nitrate was mixed with the powder". We figured they just wanted a solubility observation

That was the question! We listed all of our observations having to do with Na2S, though I don't know if we were supposed to do that.

[haven chuck]

Yeah, the question was like "Besides a rotten egg odor, what did you observe when ferric nitrate was mixed with the powder". We figured they just wanted a solubility observation

That was the question! We listed all of our observations having to do with Na2S, though I don't know if we were supposed to do that.

They said to only put down a max of two observations per question

Let me say this, pH does not tell you whether it is strong or weak acid, unless you know the concentration of the solute in the solution as in that case the H+ concentration you get from the pH (if the pH was 4 the H+ concentration would be .0001 mol/L) matches the total concentration of the solute in the solution you have a strong acid. If it were less then the total concentration it would be weak.

That's true, but most supervisors won't know/expect you to know that, and will just ask if the solution was acidic, basic, or neutral (this has happened on almost all of the past nationals tests if I'm correct). That being said, any extra information is good to learn, since they can pretty much ask anything about chem relating to something being used.

[starpug]

Let me say this, pH does not tell you whether it is strong or weak acid, unless you know the concentration of the solute in the solution as in that case the H+ concentration you get from the pH (if the pH was 4 the H+ concentration would be .0001 mol/L) matches the total concentration of the solute in the solution you have a strong acid. If it were less then the total concentration it would be weak.

That's true, but most supervisors won't know/expect you to know that, and will just ask if the solution was acidic, basic, or neutral (this has happened on almost all of the past nationals tests if I'm correct). That being said, any extra information is good to learn, since they can pretty much ask anything about chem relating to something being used.

All you need is hydrion paper to tell that, despite its inadequacies it should show basic as basic and acidic as acidic. I guess my point was they couldn't ask you strong or weak without giving you more information such as concentrations or the name of the compound (in which case you would use the strong acid list).

[frogzorz]

Yeah, the question was like "Besides a rotten egg odor, what did you observe when ferric nitrate was mixed with the powder". We figured they just wanted a solubility observation

That was the question! We listed all of our observations having to do with Na2S, though I don't know if we were supposed to do that.

They said to only put down a max of two observations per question.

What?! Ugh. Well anyways, I think the powder turned yellowish with the ferric acid. That was probably the required observation.
BTW, what was your tiebreaker observation for? Ours was really bad. It was about the litmus paper and stuff.

[haven chuck]

What?! Ugh. Well anyways, I think the powder turned yellowish with the ferric acid. That was probably the required observation.
BTW, what was your tiebreaker observation for? Ours was really bad. It was about the litmus paper and stuff.

Ours was a precipitation reaction i think; something along the lines of- "When [one of the chemicals, i don't remember which] was added to the aqueous solution of the powder, a translucent, dull brown precipitate forms"

[melody2k6]

What?! Ugh. Well anyways, I think the powder turned yellowish with the ferric acid. That was probably the required observation.
BTW, what was your tiebreaker observation for? Ours was really bad. It was about the litmus paper and stuff.

Ours was a precipitation reaction i think; something along the lines of- "When [one of the chemicals, i don't remember which] was added to the aqueous solution of the powder, a translucent, dull brown precipitate forms"

But didn't the powder form precipitates in basically everything? It didn't completely dissolve in everything, but since it didn't react that much, I wouldn't call it formed precipitates. But I'm really not sure. The powder we got at State was a regular white-ish powder like the kind you have to identify in Crime Busters (it was Calcium Carbonate, I think) and it didn't give us much trouble.

[frogzorz]

But didn't the powder form precipitates in basically everything? It didn't completely dissolve in everything, but since it didn't react that much, I wouldn't call it formed precipitates. But I'm really not sure. The powder we got at State was a regular white-ish powder like the kind you have to identify in Crime Busters (it was Calcium Carbonate, I think) and it didn't give us much trouble.

Wow, your States powder is BOORRIINNG. I heard that the Pennsylvania powder was like the Nationals one, except they gave you a propane tank with a blowtorch at PA States for observations. The powder's crystal form turned green in the flame, so the powder probably contained some form of copper.

[haven chuck]

What?! Ugh. Well anyways, I think the powder turned yellowish with the ferric acid. That was probably the required observation.
BTW, what was your tiebreaker observation for? Ours was really bad. It was about the litmus paper and stuff.

Ours was a precipitation reaction i think; something along the lines of- "When [one of the chemicals, i don't remember which] was added to the aqueous solution of the powder, a translucent, dull brown precipitate forms"

But didn't the powder form precipitates in basically everything? It didn't completely dissolve in everything, but since it didn't react that much, I wouldn't call it formed precipitates. But I'm really not sure. The powder we got at State was a regular white-ish powder like the kind you have to identify in Crime Busters (it was Calcium Carbonate, I think) and it didn't give us much trouble.

I think you may not understand what a precipitate is. If a powder is added to a liquid and it doesn't completely dissolve, then the powder is partially soluble, or insoluble if it doesn't dissolve at all. However, that has nothing to do with precipitates. If two liquids are mixed (i.e. the aqueous solution of the powder and one of the chemicals) and a solid forms (it should be fairly obvious that it is not a normal solution), then that is a precipitate. A powder added to a liquid could never form a precipitate, by definition. Also, if a liquid remains above the precipitate, that is called a supernate (just more information that might be helpful). As far as the chemistry behind it, this might help- http://wiki.answers.com/Q/Why_do_precipitates_form

But didn't the powder form precipitates in basically everything? It didn't completely dissolve in everything, but since it didn't react that much, I wouldn't call it formed precipitates. But I'm really not sure. The powder we got at State was a regular white-ish powder like the kind you have to identify in Crime Busters (it was Calcium Carbonate, I think) and it didn't give us much trouble.

Wow, your States powder is BOORRIINNG. I heard that the Pennsylvania powder was like the Nationals one, except they gave you a propane tank with a blowtorch at PA States for observations. The powder's crystal form turned green in the flame, so the powder probably contained some form of copper.

Yeah, the PA states test was the most oddball test imaginable. Propane tank, no HCl, iron nail soaked in aqueous solution, and questions about anhydrates and pentahydrates