The AP Chemistry Study Thread

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81

<p>sristi, you don't need the volume.
your looking at equivalence point, where moles of CH3NH2 = moles of HCl.</p>

<p>M and moles, solve for volume.</p>

<p>50mL @ .1M CH3NH2 moles = .1*(.05) = .005 moles CH3NH2
.2M HCl and .005 moles = .005/.2 = .025L = 25mL.
Ch3NH2 + HCl --> CH3NH3+ + Cl-
.005 .005 0 0
-.005 -.005 +.005 +.005
0 0 .005 .005
^all moles^</p>

<p>new volume is 50ml + 25ml = 75mL. 005moles/.075L = .0666666666667M CH3NH3+</p>

<p>CH3NH3+ <----> CH3NH2 + H+
Ka = 2.3E-11 = [CH3NH2][H+]/[CH3NH3+]</p>

<p>[CH3NH2]=[H+]=x</p>

<p>2.3E-11 = x/.066666666666666666667
x = 2.3E-11*.066667 = some number
-log(x) = PH
don't have grapher on me, but there is the work.</p>

82

<p>Thanks. Didn't notice the part about the equivilance point. I got 5.9 for the PH</p>

<p>An ion containing only oxygen and chlorine is 31% oxygen by mass. What is its empirical formula?
NO CALCULATORS</p>

<p>A) Cl0
B) Cl02
C) ClO3
D) ClO4
E) Cl2O</p>

83

<p>well Cl2O doesn't exist, so that's out.
31% oxygen means out of 100grams, 31 are oxygen. means 69g are clorine.</p>

<p>31g about equal to 32, which is 2 moles. 69 close enough to 71, which is 2 moles of Cl.
2moles of Cl
2moles of O</p>

<p>answer is A) ClO</p>

<p>let me know if i'm right.</p>

84

<p>new question please</p>

85

<p>EvilBooyaa~ you are right. give us a new questions</p>

86

<p>meh not a fan of giving questions cause I make them unnecessarily hard.
but fine.</p>

<p>I-131 decays by emitting an alpha particle and an electron.
Write out the full equation representing the statement above.</p>

87

<p>I-131-53 --->electron-0-(-1) + alpha-4-2 + Te-127-52.</p>

<p>correct me if im wrong, we're still on radiation (last section).</p>

<p>Balance the following half-rxn:</p>

<p>MnO4- ----> Mn+2</p>

88

<p>MnO4- + 8H+ + 5e- ---> Mn 2+ + 4H2O</p>

<p>In a system that is at dynamic equilibrium:
a) increasing the temperature of the system will drive the reaction in which direction?
b) increasing the pressure of the system will drive the reaction in which direction?</p>

89

<p>a) drive the reaction forward
b) drive the reaction forward - less moles</p>

<p>What is the pH of Acetic Acid (50 mL, .10 M) when it is titrated with NaOH (5 mL, .15 M)?</p>

90

<p>theautumneffect~ I don't think question a) has a logical answer. We need to know if the system is endothermic or exothermic.
b) It moves to the side with the least number of moles. </p>

<p>Scareya~ I got 3.22 for the PH. Using the acid dissasociation constant I found the H concentration. Then found the number of moles of H. O then substracted the number of moles OH from it. Then used .055 L and found the molarity. Then used -log (H) to find the PH. I think I am wrong, please correct me. Thanks</p>

<p>My question, anyways~</p>

<p>Define isomers:</p>

91

<p>sristi - thanks for pointing that out. i got the question from the AP chem powerpack thing, and the answer they give for a) is:
"Increasing the temperature of such a system will drive the reaction in the exothermic direction, whether that is toward the creation of more reactants or of more products. This is because the heat energy put into the system can be used to drive the exothermic reaction."
You're right, you have to mention that whether the reaction is exo or endo, but instead of the question asking "in the case of an exo reaction..." i think they want to hear you state under what conditions, like in b) where you have to say it will move to the side with fewer moles. sorry if the question was kind of ambiguous.</p>

92

<p>Isomer - One of two or more forms a chemical compound which have the same number and type of each atom but a different arrangement of atoms.</p>

<p>I forgot to say the Ka for acetic acid is 1.8 x 10^-5.</p>

<p>Weak Acid Titrated with a Strong Base</p>

<ol>
<li>Convert everything to moles</li>
</ol>

<p>Acetic acid
(50 mL)(1 L / 1000 mL)(.10 mol/ 1 L) = 0.005 mol </p>

<p>NaOH
(5 mL)(1 L/ 1000 mL)(.15 mol/ 1 L) = 0.00075 mol</p>

<ol>
<li>Stoichiometry
____<strong><em>HC2H3O2 (aq) + OH- (aq) -> C2H3O2- (aq) + H20 (l)
Before rxn .005</em></strong><strong><em>.00075</em></strong><strong><em>0</em></strong>________-</li>
</ol>

<p>OH- is clearly the limiting reactant.</p>

<p>After rxn .000425____<strong><em>0</em></strong><strong><em>.00075</em></strong>_____-</p>

<ol>
<li><p>Calculate the total volume
50 + 5 = 55 mL = 0.055 L</p></li>
<li><p>Calculate the new concentrations
[HC2H3O2] = (.000425 mol / .055 L) = 0.00727
[C2H3O2-] = (.00075 / .055 L) = 0.01361</p></li>
<li><p>Equilibrium Calculation
HC2H3O2 (aq) + H20 (l) <-> H+ (aq) + C2H3O2 (aq)</p></li>
</ol>

<p>Ka = [H+][C2H3O2] / <a href="1.8%20x%2010%5E-5">HC2H3O2</a> = <a href="0.01361">H+</a> / (0.00727)</p>

<p>[H+] = 9.61 </p>

93

<p>How to do titrations with a weak acid and a strong base:
1. Convert to moles
2. Neutralization: HX + OH- -> X- + H20
The OH- comes from the strong base ionizing completely in water. The HX is the weak acid.<br>
3. Calculate [HX] and [X-] after reaction.
4. Use Ka, [HX], and [X-] to calclate [H+]
5. Calculate pH from [H+]</p>

<p>You can always use the Henderson Hasselbalch equation:
pH = pKa + log([base]/[acid])
pKa = -log(ka)</p>

<p>How to do titrations with a strong acid and strong base:
1. Convert to moles
2. Neutralization: H+ + OH- -> H20
The H+ comes from the strong acid ionizing completely in water. The OH- comes from the strong base ionizing completely in water.
3. Calculate [H+] and [OH-].
4. Calculate pH from [H+]</p>

94

<p>scareya~how do you use the henderson equation? This is what I did but I came out with a different answer than using the ice diagrams. (by the way, I got 5.5 for the PH. I used like 15 sig figs.)
pH = pKa + log([base]/[acid])
[base]= (.15 M*.005 L)/.055L= .013636
[Acid]= (.10 x .05L)/.055 =.090909
= -log[1.8 x 10^-5.]+ log ( .013636/.090909)
= 3.92
I don't know how that works out. Please help. </p>

<p>For sig figs:
for division and multiplication, I remember that LESS always wins
e.g 12.000 (5 sig figs) x 1.0 (2 sig figs) = 12 (2 sig figs)</p>

<p>For addition and substraction, I remeber that LESS always wins also. But this time it is the decimal place that counts.<br>
e.g. 12.000 (3 decimal places) + 1.0 (1 decimal place)= 13.0 (1 decimal place) </p>

<p>I guess LESS always wins in sig figs </p>

<p>My question:
Which of the following species is amphoteric?
A) HNO3
B) HC2H3O2
C) HSO4
D) H3PO4
E) ClO4</p>

95

<p>sristi: I think you are using the wrong concentrations for the acid and base.</p>

<p>I think you were using the initial concentrations in the HH equation, which is incorrect.</p>

<p>pH = pKa + log([base]/[acid])</p>

<p>pKa = -log[1.8 x 10^-5]
[base] = 0.01361 M
[acid] = 0.00727 M</p>

<p>pH = 5.03</p>

<p>The HH equation only allows you to bypass the last 2 steps, which are the equilibrium and pH calculation. It combines both of those steps together.</p>

<p>Please correct me if I'm wrong.</p>

96

<p>Thanks scareya. I get it now. anyone want to give my Q a try</p>

97

<p>An amphoteric species is one that can act both as an acid or base depending on the situation. Water is a perfect example of a amphoteric species. It acts like an acid when it is put in a basic solutions and a base when put in a acidic solution. </p>

<p>It has to do with the molecular structure...I'll make a post if I can figure it out.</p>

98

<p>I think its H3PO4. I was looking through my Chem textbook and it has a list of bases and acids....and H3PO4 was in both lists.</p>

<p>How many significant figures does 130 cm have?</p>

99

<p>130 has 2 sig figs. i forgot to add, you can buy an Application for the texas instrument graphing calculators which does sig figs. It came with my TI 84 silver.
Anyone else want to answer my question. It is not correct</p>

100

<p>The answer is C. I asked my friend. Usually polyprotic acids are amphoteric.</p>

<p>HSO4 can be an acid and become SO42- or be a base and become H2SO4</p>