aamc 8, PS # 38

[rebel1]

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Can someone please help me understand this question?

If red litmus paper is dipped into the Na2CO3 solution, it will:

a) remain red, becuase carbonate is an acidic salt
b) remain red, becuase sodium carbonate is neutral
c) turn blue, because carbonate reacts with water to produce OH-
d) turn blue, because sodium ions reacts with water to form sodium hydroxide in water

the correct answer is c
I thought it would be d 🙁

 

[rls303]

Can someone please help me understand this question?

If red litmus paper is dipped into the Na2CO3 solution, it will:

a) remain red, becuase carbonate is an acidic salt
b) remain red, becuase sodium carbonate is neutral
c) turn blue, because carbonate reacts with water to produce OH-
d) turn blue, because sodium ions reacts with water to form sodium hydroxide in water

the correct answer is c
I thought it would be d 🙁

Hi,
I'm not entirely sure, but I think the CO3 is what deprotonates the water and produces the OH.

With regards the red litmus paper, do we have to know the different types of indicators for MCAT?

 

[PiBond]

Can someone please help me understand this question?

If red litmus paper is dipped into the Na2CO3 solution, it will:

a) remain red, becuase carbonate is an acidic salt
b) remain red, becuase sodium carbonate is neutral
c) turn blue, because carbonate reacts with water to produce OH-
d) turn blue, because sodium ions reacts with water to form sodium hydroxide in water

the correct answer is c
I thought it would be d 🙁

CO3 is a moderately strong base so eliminate A. recall that it's the conjugate base of a weak acid (carbonic acid--H2CO3), eliminate B. we know that bases of its strength will react with water to produce OH-....sodium doesn't react with water so eliminate D. Na+ is only solvated and has no basic properties.

 

[mmcoffee]

Can anyone explain why Na+ wouldn't react with water? Have a weak chem background

 

[NextStepTutor_3]

Choice D incorrectly implies that Na+ can deprotonate a water molecule, since it mentions the formation of sodium hydroxide. In other words, for D to be true, Na+ would need to be a Bronsted-Lowry base, which it is not. Na+ is neutral (according to the Bronsted-Lowry definition) - it neither protonates nor deprotonates any other compounds, not just water. CO2 3-, on the other hand, is a fairly good base, since it's the conjugate of HCO3- (bicarbonate). Bases accept protons, and the carbonate ion will remove a proton from H2O.

As for the indicators, you definitely don't need to know the pH ranges of certain ones, but it's helpful to understand what the various colors of litmus paper mean. Red litmus paper will remain red in acidic solution and turn blue in basic solution.

Good luck!