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Acids and Bases P1 Help!

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28. These salts have anions that are conjugate bases of some acids. NO3-comes from HNO3, SO42- comes from HSO4-, Cl- from HCl, and Br- from HBr. Now for weak acids, conjugate bases are (relatively) strong bases. HSO4- is the only weak acid and Na is a Group I metal, meaning the compound will dissociate in water, that means the strongest base SO42- (out of the 4) will increase the pH. B

29. Buffer solution is when there is same number of moles (equimolar) of a weak acid as its conjugate base or weak base and its conjugate acid. HCl and NaOH, and KCl are not weak acids/bases. so we need the other species in the mixture to be at twice the mol. In D, 0.10 mol of NH3 will react with HCl to form NH4+, that would make 0.10 mol of NH3 and 0.10 mol of NH4+ present, satisfying the condition for buffer solution. Experimentally, they don't have to exactly the same, but usually as long as the resulting pH is within ± 1 pH unit of the equimolar pH, the solution is considered to be a buffer solution. Note that in the other 3 choices, all of the weak acid/weak base will react, leaving no weak acid/base behind. Finally, strong acids/bases are not present in a buffer solution because the conjugate is too weak to impact pH (see question 28). 

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8 hours ago, Scienuk said:

May anyone please answer these questions in detail? thank you!

The answers are A and D, respectively. 

Screen Shot 2017-04-07 at 11.23.13.png

 

are these HL or SL chem questions? I am confused - is it normal that I did not know the answer of either? I take SL Chem.

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1 minute ago, Amina13 said:

are these HL or SL chem questions? I am confused - is it normal that I did not know the answer of either? I take SL Chem.

They're HL, you're fine :)

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1 hour ago, Amina13 said:

phew, what an exam stress! :)

I feel like I don't know anything and only days left to the exams - not studying enough :( 

Don't worry! the exam is on the 11th, more than a month left!

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1 hour ago, kw0573 said:

I thought 28 was B. I guess since the answer is A, it's because it has most number of anions. 

I second this. 28 should be due to the the amount of NO3- ions per each mole. 

Al3+ + H2O --> Al(OH)2+ +H+

Please correct me if I'm wrong, but the reaction which gives more H3O+ or H+ changes the pH the most (basing it off the Bronsted-Lowry's theory of acids and bases).

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28. The aluminum ion has a high charge density and is able to hydrolyse water molecules to produce H+ ions in solution and a decrease in pH. 

29. A buffer solution should have equal amounts of a weak acid and the salt of the weak acid and strong base. Whichever choice gives you that is the correct answer.

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