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### Topic: Acids/Bases - Losing H, does it matter which?  (Read 8362 times)

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#### Loyal

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##### Re: Acids/Bases - Losing H, does it matter which?
« Reply #15 on: February 22, 2009, 04:08:19 PM »
Just look at the units.  If you divide mol/L by L doesn't that make it mol/L2

The problems asks for the OH concentration and the subsequent pH.   Since it does not ask for activity coefficients the sodium and the barium concentrations are irrelevant.   You only care about how much hydroxide is in solution.  So for Ba(OH)2 there are 0.030 mols per liter of OH and for NaOH there is 7.5x10-3 moles per liter.    You added 10mL of Ba(OH)2 so that means there are  0.030M*(10/1000)L moles of hydroxide from that source and (7.5x10-3)M*(40/1000)L moles of hydroxide from the NaOH source.   So when you have the total number of OH moles you then divide by the new volume to find out the concentration.  That gives you the first answer and the second is rather easy to find out as you can obtain the pOH without much hassle and then you receive the pH by subtracting the pOH from 14.

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#### vort3x

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##### Re: Acids/Bases - Losing H, does it matter which?
« Reply #16 on: February 22, 2009, 05:01:52 PM »
Okay, so this is what I did. How does it look? I found the final molarity, pH, and OH- (by finding H+ and re-arranging Kw).

0.03 mol/L * 0.01 L = 0.0003 mol
7.5 x 10-3 mol/L * 0.04 L = 0.0003 mol
+ = 0.0006 mol

0.0006mol / 0.05 L = 0.012 mol/L

pOH = -log(0.012mol/L) = 1.92
pH = 14.00 - 1.92 = 12.08

[H+] = 10-12.08 = 8.32 * 10-13

[OH-] = 1.00 * 10-14 / 8.32 * 10-13 = 1.2 * 10-2
« Last Edit: February 22, 2009, 05:13:28 PM by vort3x »

#### Loyal

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##### Re: Acids/Bases - Losing H, does it matter which?
« Reply #17 on: February 22, 2009, 05:17:45 PM »
Yep that looks very close to the answer I got so I would say you have arrived at a reasonable answer.   Numerically it makes sense that a dilute solution of hydroxide would have a pH of around 10 to 12.

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#### vort3x

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##### Re: Acids/Bases - Losing H, does it matter which?
« Reply #18 on: February 22, 2009, 05:23:28 PM »
What answer did you get, do we have the same numbers? Just want to make sure I'm rounding everything properly too.

#### Loyal

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##### Re: Acids/Bases - Losing H, does it matter which?
« Reply #19 on: February 22, 2009, 05:29:04 PM »
What answer did you get, do we have the same numbers? Just want to make sure I'm rounding everything properly too.

They look identical.   I assume you are taking a first year college or high school Chemistry course so there should be no corrections you have to apply.
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