<p>Since most of us taking the AP chem are going to try this test out, lets start a discussion on the problems here!
Can someone explain the answer to #12 and #65? I didnt understand those two...</p>
<p>I would like to know how to solve those two as well. I know for #65 you can solve it simply by process of elimination (I think all of the other options are soluble in water), but I would like to know why Mg(OH)2 is soluble in HCl, but not in H2O.</p>
<p>For those that don’t feel like going to the released exam online, #12 is the following:</p>
<p>12) A buffer at a pH < 6.
A. NH3 and NH4Cl
B. H3PO4 and NaH2PO4
C. HCl and NaCl
D. NaOH and NH3
E. NH3 and HC2H3O2</p>
<p>Also, could anyone explain to me what the purpose of a spectrophotometer is? We used it in class before but I forgot for what…</p>
<p>I believe the answer would be C because that is the only answer choice containing a strong acid (HCl) which would obviously lower the pH.</p>
<p>A buffer only occurs with a weak acid and its conjugate base or a weak base and its conjugate acid. The answer is B.</p>
<p>The answer is B, but why isn’t it C is what I’m wondering.</p>
<p>My fault, I didn’t see the “buffer”, only saw the pH of 6. THe answer is B.</p>
<p>Let me see if I understand then… tell me if I’m doing everything wrong please.</p>
<p>HCl and NaCl is not even a buffer because the Cl- in the NaCl will not react with any added acid to form HCl and keep the H+ concentration at the same spot. This is because HCl is a strong acid and dissociates completely. H+ and Cl- forming HCl is pretty much non-existent. (Is the reason this option is not correct because it isn’t a buffer?)</p>
<p>H3PO4 and NaH2PO4 is a buffer, and an effective one at that, because the H2PO4- will react with any added H+ to form H3PO4 as the backward reaction is more favored than the forward one. This will keep the H+ concentration at approximately the same level.</p>
<p>What happens if OH- is added though (I think I know what I have to study today and tomorrow…)?</p>
<p>How do you know that NaH2PO4 is not a stronger base than H3PO4 an acid?</p>
<p>woahh there. A buffer is a weak acid/base and its salt.</p>
<p>isnt a buffer a weak acid and its conjugate base in equivalent amount?</p>