Chemistry FR

Test Preparation AP Test Preparation
101

<p>Here's how my teacher did the ph question:</p>

<p>Using the reaction, he found .01 mol of HF and .006 mol of OH-. Then, you yield .006 mol of F- ions which is a concentration of .15 M with additive volumes.
Next you write the reaction of F- ions with water yielding HF and OH- ions.
Next, find the Kb, because it's a base reaction, which is 1.39 X 10^-11
Set the Kb equal to the concentration of the OH- times the concentration of F-, each of which are X, over the concentration of F-, which is .15. Find the OH concentration take the negative long of that, and then subtract that number from 14 giving you a pH of 8.16.
Not sure if that's right, but that's the way my teacher did it in class today.</p>

102

<p>G = -RT ln(K) does work. You get 950K.</p>

103

<p>how do you get that?</p>

104

<p>Isnt K = 1 though? and the ln of 1 is 0.</p>

105

<p>ln 1 = 0, so dG = 0, then just use dH and dS to solve.</p>

106

<p>solve for what? you are solving for T, and you multiply it by 0. T disappears.</p>

107

<p>Uh... 0 = dH - TdS
-dH/-dS = T</p>

108

<p>Hmm I substituded G in that equation and got -300,000 something = 0.....what was I thinking?</p>

109

<p>still don't get it...</p>

110

<p>ahh, wait, now i get it. darn. thats weird.</p>

111

<p>pH should be around 3...</p>

112

<p>yeah, 3.32. </p>

<p>Also, what is the deal with units? did you have to put units? if you put them and they are wrong you get the point off right?</p>

113

<p>Does the pH need to be exactly 3.32? What if you're a few decimal points off?</p>

114

<p>for a problem like 2a, where dH is in kJ and dS is in J, they usually take off if units aren't there cause your answer could be in either</p>

115

<p>BobbyBrown, if you made a mistake in 5di, you lose the point there, but if the same mistake is carried through ii and iii, you shouldn't lose those also</p>

116

<p>can someone explain the temp one? i got 0 K but i dont think thats right lol,,how did you guys get the 900 answer?</p>

117

<p>hzbball - the method your teacher used is correct IF you are at the equivalence point (no H+ left). Since there is H+ and F- present, Henderson-Hasselbalch is the way to go.</p>

118

<p>Im use to 3 decimal places from Calculus, but I think my teacher mentioned if you're off by +-1 decimal, youre fine. Theres like a rule for pH and molarities...like the number of sigfigs in the molarity is the number of decimals in the pH</p>

119

<p>Much love
dG = 0 = -264 - (T)(-.278)</p>

120

<p>you have to use SIGNIFICANT FIGURES!!! thats a big part of chem. but, they are really lenient. you can be +/- 1 sig fig, or +/- 2 on pH problems (because its a logarithmic scale).</p>