puffylover

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so i've come across these questions in acid base and oxidation reduction questions. how do you know what gas evolves out of solution?
 

loveoforganic

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Your question is too broad as stated, at least to me. Could you either elaborate or give an example?
 

joshto

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the components that form the gas must be present in the solution.

for example, CO2 gas cannot evolve from a solution in which no Carbon is present; just like H2 gas cannot evolve when no Hydrogen is present in solution; N2 cannot evolve when no Nitrogen is present in the solution; etc, etc.
 

puffylover

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A class is given a demonstration of solution chemistry and solubility equilibria.

A 6.57-g sample of NiSO4•6H2O (molar mass 262.84) is dissolved in enough water to make 50.00 mL of a green solution, and 7.15 g of Na2CO3•10H2O (molar mass 286.14) is dissolved in enough water to make 50.00 mL of a colorless solution. The two solutions are mixed, and a green precipitate forms. The resulting slurry is divided into two equal portions. To one portion is added an excess of 6 M HCl, which results in the disappearance of the precipitate and a rapid evolution of a gas. To the second portion is added a few milliliters of 6 M NH3. The precipitate dissolves, and a blue solution forms.


Item 40


Solution

Mark According to the information in the passage, the gas that evolves is: A
) sulfur dioxide. B
) sulfur trioxide. C
) carbon dioxide.
According to the passage, the gas is given off when HCl is added to one portion of the slurry. HCl reacts with CO32– to form CO2(g). Thus, C is the best answer.


D
) carbon monoxide.
 

loveoforganic

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First reaction

NiSO4 + Na2CO3 ==> NiCO3 + Na2SO4

Net reaction

Ni++(aq) + CO3--(aq) ==> NiCO3(s)


Second reaction

NiCO3 + HCl ==> NiCl2 + H2CO3

In aqueous solution, H2CO3 decomposes to CO2 and H2O. This is just a fact you should know. It's the same vinegar/baking soda experiment principle everyone has seen.
 

Sarahka74

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First reaction

NiSO4 + Na2CO3 ==> NiCO3 + Na2SO4

Net reaction

Ni++(aq) + CO3--(aq) ==> NiCO3(s)


Second reaction

NiCO3 + HCl ==> NiCl2 + H2CO3

In aqueous solution, H2CO3 decomposes to CO2 and H2O. This is just a fact you should know. It's the same vinegar/baking soda experiment principle everyone has seen.
How did you know what to drop out in the net reaction?
 
Jan 17, 2018
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@Sarahka74

A 6.57-g sample of NiSO4•6H2O (molar mass 262.84) is dissolved in enough water to make 50.00 mL of a green solution, and 7.15 g of Na2CO3•10H2O (molar mass 286.14) is dissolved in enough water to make 50.00 mL of a colorless solution. The two solutions are mixed, and a green precipitate forms. The resulting slurry is divided into two equal portions. To one portion is added an excess of 6 M HCl, which results in the disappearance of the precipitate and a rapid evolution of a gas. To the second portion is added a few milliliters of 6 M NH3. The precipitate dissolves, and a blue solution forms.

Regardless of the relative solubilities of Nickel (II) Carbonate and Nickel (II) Sulfate, we expect dissociated Carbonate to react with the added HCl to form H2CO3. As mentioned earlier, H2CO3 decomposes into H20 and CO2, a process that is further driven by the high [H+]/low pH in solution (recall the bicarbonate buffer in the blood etc.).

Essentially, HCl reacts with CO3 2-, reducing the [CO3 2-] concentration. By Le Chateliers principle, the nickel carbonate will dissociate to reestablish equilibrium (hence the disappearance of ppt, and evolution of gas).

There are 2 salts to consider.
NiCO3 <--> Ni 2+ + CO3 2-
NiSO4 <--> Ni 2+ + SO4 2-


edit: I somehow confused some ****.

The beauty of the MCAT is you do not need to know the identities of the precipitates unless explicitly asked for them, in which case you would be furnished with basic data/info within the scope of relevant MCAT knowledge. In this instance, I would assume that nickel carbonate is the first ppt formed because carbonate salts typically exhibit limited solubility in aqueous solution, while sulfates are slightly more soluble. I would assume the second ppt is nickel sulfate, based on the color (you do NOT need to know how or why, and that is my GUESS). This would be beyond the scope of the MCAT with respect to solubilities and their physical/chemical properties, which is of course why no such questions were asked.

I would also attempt to memorize the basic solubility rules, as they are more likely to be directly tested.
 
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