Squall181
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Help with Calulating pH and Concentration
Okay I have this homework that needs to be done but I am running out of ideas of what I can be doing wrong. So here are the questions and my attempts
at them if anyone can provide some insight or help I will be grateful.
1. Question one
Calculate the volume of 0.116 M HCl(aq) required to neutralize the following solutions.
all the hydroxide ions in 25.0 mL of 0.215 M KOH(aq)
1. What is the molarity of Cl - ions at the stoichiometric point?
HCl + KOH --> KCl + H2O
My attempt to solve this thing
so at the stoichiometric point the amount of acid and base moles are equal
so .215M*.025L=.005375 moles of base that needs to be neutralized
so .005375n/.116M=.0463L
My theory is that the amount of Cl- ion in the solution is equal to the amount of moles of acid used since KCl is formed in the reaction and KCl
dissociates into K+ and Cl- ions completely.
So then the molarity of Cl- at the stoichiometric point is
.005375 moles/(.0463L + .025L)=.0754M
my second problem
2. Calculate the pH of the solution after the addition of 40.0 mL of 0.116 M HCl(aq) to 25.0 mL of 0.215 M KOH(aq).
.04L*.116M=.00464moles
.025L*.215M=.005375moles
so we subtract .005375n-.00464n=0.000735 moles of KOH left
no we get concentration by dividing .000735moles/(.04+.025)L=.01131 Molar
to get pH we first calculate the pOH since we still have base present
-log(.01131)=1.9466
pH= 14 - 1.9466 = 12.0533
I don't understand why this is the wrong answer seems like I did all the other problems the same way and got a right answer, but here I don't get it.
If you guys can please take a look and see if I am missing something I would greatly appreciate your help.
Thank You
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barley81
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Your calculations seem correct, but you have a problem with significant figures.
Here is a link to a website which explains them:
web.campbell.edu/faculty/fetterman/Significant%20Figures.htm
The minimum number of sig. figs in the data you are given is 3. All of your answers should be 3 sig. figs, but do not round intermediate numbers for
better accuracy. Logarithms have different rules, so read about it on the site. Hope this helps!
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Squall181
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Thank you for the link that is a good refresher on sig figs.
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