Smooth Operater

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choices are:
a) NaCl
b) LiF
c) Na2S
d) KBr
e) BF3

the correct answer is c. can anyone go through each compound and rationalize why? Thanks!!
 

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Smooth Operater said:
choices are:
a) NaCl
b) LiF
c) Na2S
d) KBr
e) BF3

the correct answer is c. can anyone go through each compound and rationalize why? Thanks!!
Basicity increases as electronegativity decreases, and increases up the group. using that logic, we can isolate to answers b and c. between LiF and NaS2, i would pick NaS2 because it requires 2 group 1A metals to balance it out.
 

clemsontigers

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Envision said:
Basicity increases as electronegativity decreases, and increases up the group. using that logic, we can isolate to answers b and c. between LiF and NaS2, i would pick NaS2 because it requires 2 group 1A metals to balance it out.
i would think it would be between c and d using that logic.
 
B

BentalScholar

Smooth Operater said:
choices are:
a) NaCl
b) LiF
c) Na2S
d) KBr
e) BF3

the correct answer is c. can anyone go through each compound and rationalize why? Thanks!!
A weak acid produces a strong base. Thus HCl, HF, HBr are strong acids thus they give weak bases. Flourine is a very electronegative atom and thus it is least likely to act as a base and donate electrons. On the other hand, S has a d-orbital that is more likely to lose the electrons from more easily than those in BF3, not to forget that Na2S is an ionic compound and thus ionic bonds are easier to break than covalent ones in BF3 thus making the Na2S a better base than all the others. I hope this explains it well.
 

allstardentist

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The first four choices are all salts i believe and the last one BF3 is a lewis acid(I just know it, maybe b/c it has no lone pair). So we can eliminate E. And now we have the four salts. Cl,F,Br are all conjugate base of strong acid(HCl,HBr,...). So they will be weak bases. So we are left with Na2S. Na itself is not a good acid in water since its a Group 1 cation and Sulfur has 2 lone pairs of e- thus it can act as a base in water. I know the kaplan gchem section is not great (got this info from Princeton Review mcat book). But like everyone says, it should be good enough for the DAT
 

thehipster

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While allstardentist's reasoning is correct, HF is not a strong acid, and thus F- would not be a weak conj base. You can eliminate F- based on what BentalScholar said about it being very electronegative.
 
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Smooth Operater

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thanks for the explanation.

But, when dealing with acid and base determination of a compound, do we always look at the 2nd element in the compound. Like in NaCl and LiF, we just look at Cl and F respectively for comparision. Can you do the same thing to other compounds?
 

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clemsontigers said:
i would think it would be between c and d using that logic.
oh whoops. ya, C & D, thanx for pointing that out. also, i like to think of Na2S requires more metal to neutralize it compared to the other options.
 

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Smooth Operater said:
thanks for the explanation.

But, when dealing with acid and base determination of a compound, do we always look at the 2nd element in the compound. Like in NaCl and LiF, we just look at Cl and F respectively for comparision. Can you do the same thing to other compounds?
yes, you look at the conjugate cpds. for example, to compare H-I and H-F, H-I is the stronger acid, while H-F is a weaker acid. this is because conjugate base of H-F is F, and F is the smallest of all the halogens. this means the charge is very concentrated, making it very electronegative but also unstable, thus, it makes a weak acid. H-I is much bigger, and so the charge is spread over a larger amount of space, so it is more stable, making it a better acid. furthermore, F is a stronger base than I. i like to think of it like the H-I is happy with the amount of electrons it has, so it wouldnt mind giving away a proton (good acid, weak base). H-F on the other hand is unstable, so it doesnt wanna give a way a proton (weak acid), and if it loses it, it becomes a strong base because it wants the proton back. i hope this helps.
 
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Smooth Operater

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I know this may be too detailed but I would like to know why charge spreading over a larger amount of space with greater stability would make strong acid? And why unstability makes weaker acid? Can you explain in the context of Lewis acid/base (i.e. acid receive electrons, base dontes electrons) ? Thanks!