Help with chemistry questions (1 Viewer)

[Vaibhav123456]

The answers are: 2(B), 6(B), 8(C). Just want to know what the reasoning behind the answers is. For Q8 is it because NO2- is a stronger conjugate base and hence, has a higher pH, and for Q6 is it because phosphoric acid is triprotic. Got no idea about Q2, maybe because there is no proton donor.
Thanks in advance.

 

[Qeru]

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The answers are: 2(B), 6(B), 8(C). Just want to know what the reasoning behind the answers is. For Q8 is it because NO2- is a stronger conjugate base and hence, has a higher pH, and for Q6 is it because phosphoric acid is triprotic. Got no idea about Q2, maybe because there is no proton donor.
Thanks in advance.

2NH3=NH3+NH3 so A works. No such trick with B

 

[Vaibhav123456]

2NH3=NH3+NH3 so A works. No such trick with B

Thanks. Are the other two right then.

 

[Eagle Mum]

Yes, your understanding is correct.

For Q8, to explicitly explain how the provided information on pKa is relevant.

HNO2 + NaOH -> NaNO2 + H2O
HClO2 + NaOH -> NaClO2 + H2O

Since strong bases (NaOH) plus weak acids produce salts with greater pH, we want to work out which is the weaker acid out of HNO2 & HClO2.

Since pKa is the dissociation constant = -log ([Conj. Base][H+]/[Acid]), the higher pKa of HNO2 indicates that it dissociates less than HClO2 and hence HNO2 is the weaker acid, so the corresponding salt, NaNO2 has the higher pH.

 

[CM_Tutor]

Question 2: To be a L-B A/B reaction, there must be a transfer of one proton from the L-B acid to the L-B base.

In A, the ammonium ion transfers 1 proton to the amide ion.

In C, the perchloric acid transfers 1 proton to the acetic acid.

In D, the methylammonium cation transfers 1 proton to the ethoxide ion.

There is no 1 proton transfer in B.

Question 6: Strength is not a factor in the neutralisation, only the concentrations and volumes of the acid and base and the stoichiometry. As you noted, phosphoric acid is triprotic whereas all the others are monoprotic.

Question 8: The pK_a values tell you that chlorous acid is stronger than nitrous acid. It follows that the chlorite ion (pK_b = 12.04) must be a weaker base than the nitrite ion (pK_b = 10.61). So, for equal concentrations, we can compare the pH's directly:

pH (chlorous acid) < pH (nitrous acid) < pH (chlorite salt) < pH (nitrite salt)

And, given we have the concentrations are all 0.1 M, we can even calculate the pH values:

pH (chlorous acid) = 1.48 < pH (nitrous acid) = 2.195 < pH (chlorite salt) = 7.48 < pH (nitrite salt) = 8.195