NiSO4 + Na2CO3 + HCl --> What gas?

[zut212]

The question under AAMC #8 reads (I'm editing out all the unimportant stuff):

NiSO4*6H2O is dissolved in enough water to make a green solution, and Na2CO3*10H2O is dissolved to make a colorless solution. The two solutions are mixed, and a green precipitate forms. The resulting slurry is divided into 2 equal portions. To one portion is added an excess of 6M HCl, which results in the disappearance of the precipitate and a rapid evolution of a gas.

According to the passsage, the gas that evolves is:
A. sulfur dioxide
B. sulfur trioxide
C. carbon dioxide
D. carbon monoxide













CORRECT is C. According to the passage, the gas is given off when HCl is added to one portion of the slurry. HCl reacts with CO3(-2 superscript) to form CO2(g).


However, there are many permutations/combinations of things released.
NiCO3
Na2SO4
NaSO4-
NaCl
H2CO3
HCO3-
H2SO4
HSO4-
NiCl2
NiCl+

I could go on, but how do they figure out that CO2 is being released?

---

Members don't see this ad.

 

[BerkReviewTeach]

The question under AAMC #8 reads (I'm editing out all the unimportant stuff):

NiSO4*6H2O is dissolved in enough water to make a green solution, and Na2CO3*10H2O is dissolved to make a colorless solution. The two solutions are mixed, and a green precipitate forms. The resulting slurry is divided into 2 equal portions. To one portion is added an excess of 6M HCl, which results in the disappearance of the precipitate and a rapid evolution of a gas.

According to the passsage, the gas that evolves is:
A. sulfur dioxide
B. sulfur trioxide
C. carbon dioxide
D. carbon monoxide

CORRECT is C. According to the passage, the gas is given off when HCl is added to one portion of the slurry. HCl reacts with CO3(-2 superscript) to form CO2(g).

However, there are many permutations/combinations of things released.
NiCO3
Na2SO4
NaSO4-
NaCl
H2CO3
HCO3-
H2SO4
HSO4-
NiCl2
NiCl+

I could go on, but how do they figure out that CO2 is being released?

You need to start by identifying the precipiate. The green solution is a fully soluble solution with Ni2+ and SO42-, with a small amount of HSO4- (from the A/B equilibrium). The clear solution is a fully soluble solution with Na+ and CO32-. Only two possible precipiates can form upon mixing the two solutions, Na2SO4 or NiCO3, with the fact that the precipitate is green hinting that it's NiCO3.

Upon adding HCl to the precipitate, it dissolves and a gas is evolved. The gas is not going to be NaCl or NiCl2, so it has to result from the reaction of H+ with CO32- (or H+ with SO42- if you weren't sure that the precipitate was NiCO3).

They expect you to know that 2 H+ + CO32- leads to H2O + CO2 based on your understanding of exhalation in physiology.

The gas really can only be CO2 from the facts presented.

As for your list, you are overthinking their question. Look at that list and think which could turn into a gas.