#62 gchem question

[whoaaitzkyle]

I don't get when I'm supposed to use M1V1=M2V2 and N1V1=N2V2...I know what molarity and normality means but yea here's the question.

Q: If 20ml of 0.12 M solution of Ca(OH)2 is added to 48ml of HBr, what s the concentration of HBr?

The method described was N1V1=N2V2. I understand the process but can someone explain to me how to immediately tell to use THAT formula and not the other?

thank you for helping me out!

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[Shunwei]

I don't get when I'm supposed to use M1V1=M2V2 and N1V1=N2V2...I know what molarity and normality means but yea here's the question.

Q: If 20ml of 0.12 M solution of Ca(OH)2 is added to 48ml of HBr, what s the concentration of HBr?

The method described was N1V1=N2V2. I understand the process but can someone explain to me how to immediately tell to use THAT formula and not the other? ehh, hope this isn't a dumb question. thanks!

I believe you need the molarity of HBr to solve this problem. If this information is available, it should be pretty straight-forward, since it would be just [conc.] HBr - 0.048 moles OH- / 0.068 L, since both acid and base species dissociate completely.

 

[AaBbZz]

The way I tell when to use normality instead of molarity is that I see OH and H. Normality is used when doing an acid base neutralization so when you see Ba(OH)2 with a molarity concentration your going to have to double it. If you see something that is H3PO4 (acid) that is .48M your going to have to triple it, etc, etc.
Just be conscious of it and look carefully, hope this helps.

 

[whoaaitzkyle]

oooooooo....thanks I get it

 

[levyusupov]

N1V1=N2V2 is used for titration and neutralization only.