Scienuk Posted April 7, 2017 Report Share Posted April 7, 2017 May anyone please answer these questions in detail? thank you! The answers are A and D, respectively. Reply Link to post Share on other sites More sharing options...
kw0573 Posted April 7, 2017 Report Share Posted April 7, 2017 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). 2 Reply Link to post Share on other sites More sharing options...
Amina13 Posted April 7, 2017 Report Share Posted April 7, 2017 8 hours ago, Scienuk said: May anyone please answer these questions in detail? thank you! The answers are A and D, respectively. 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. Reply Link to post Share on other sites More sharing options...
Scienuk Posted April 7, 2017 Author Report Share Posted April 7, 2017 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 1 Reply Link to post Share on other sites More sharing options...
Amina13 Posted April 7, 2017 Report Share Posted April 7, 2017 phew, what an exam stress! I feel like I don't know anything and only days left to the exams - not studying enough Reply Link to post Share on other sites More sharing options...
kw0573 Posted April 7, 2017 Report Share Posted April 7, 2017 I thought 28 was B. I guess since the answer is A, it's because it has most number of anions. Reply Link to post Share on other sites More sharing options...
Scienuk Posted April 7, 2017 Author Report Share Posted April 7, 2017 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! Reply Link to post Share on other sites More sharing options...
mac117 Posted April 7, 2017 Report Share Posted April 7, 2017 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 A 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). 2 Reply Link to post Share on other sites More sharing options...
Msj Chem Posted April 7, 2017 Report Share Posted April 7, 2017 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. 2 Reply Link to post Share on other sites More sharing options...
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