Dat destroyer chem problem #176 2011

[spoog74]

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Sorry.

The question asks;

30 mL of .20 M barium hydroxide is required to neutralize 25ml of citric acid, H3C6H5O7. What is the molarity of the citric acid solution?

I got did the n1v1=n2v2 and got .5M of the citric acid but then the answer is divided by 3 because " it has 3 ionizable portions"

Anyone know what that means and why we divide the final answer by 3

 

[AllahIsGreater]

the question is in regards to hess' law


Now, dont you ADD all the delta H up in order to find the final delta h of the process?

I got the answer is -250kj but the answer is -550 he SUBTRACTED the given data.


if anyone has the 2011 please help. Thanks

Hey,

Look carefully at the information given. the deltha h's that are given are all based on 2 moles of everything.

2Rh + O2 -----> 2 RhO delta H= -800
2RhO + O2 ------> 2RhO deltha H= -300

HOWEVER LOOK AT OUR "MOTHER" EQUATION:

Rh + O2 ----> RhO

that being said, we want to be like our "mother", so the data give is all twice as much as our mother so we must do the following work:

(-800/2) + (-300/2) = -400 + -150 = -550

Also recall that everything matches up, so we kept our delta H's the same, had something been switched around, we would have taken the opposite sign of delta H because we would have been looking at the reverse reaction. Hope this helps. take care.

 

[casper22]

I dont have the book in front of me right now, but I did this question yesterday, and if I remember correctly, the 2 equations have a coefficient of 2 whereas, the overall equation does not have that...so you basically divide each delta H's by 2.