GChem question

[asckwan]

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How many milliliters of 3.0 M H2SO4 are required to neutralize 30.0 ml of 4.0 M NaOH?
A. 15
B. 20
C. 25
D. 30
E. 40

This is a question on DAT Achiever test 1. I can't figure out the first part of the solution though because they just give a crap solution. "Without the need to express a full chemical equation, it is obvious that only 0.5 mole of H2SO4 is required to neutralize every mole of NaOH." I get the feeling like Michelle from Full House just said "Duh!" to me and I still can't get the calculation... whoopee...

 

[Clover]

How many milliliters of 3.0 M H2SO4 are required to neutralize 30.0 ml of 4.0 M NaOH?
A. 15
B. 20
C. 25
D. 30
E. 40

This is a question on DAT Achiever test 1. I can't figure out the first part of the solution though because they just give a crap solution. "Without the need to express a full chemical equation, it is obvious that only 0.5 mole of H2SO4 is required to neutralize every mole of NaOH." I get the feeling like Michelle from Full House just said "Duh!" to me and I still can't get the calculation... whoopee...

H2SO4 gives off 2 H's to neutralize the 1 -OH from NaOH.
You're dealing with normality.

For H2SO4: 3.0M H2SO4 = 6.0Normality --> 3 * 2 (this is # of H)
For NaOH: 4.0M = 4.0 N --> 4 * 1 (# of OH)
Then use N1V1 = N2V2

--> (6N)(V2) = (4)(30)
and solve to get B. 20ml


I haven't taken the DAT yet but I found that DAT Achiever is a good assesment tool more than a learning one. Perhaps you should try DAT Destroyer. I found out about the link on this website. Since then I've bought the book and am attending Dr. Romano's classes (I live in New York). The book is in question/answer format. It's concise and covers the wrong answers as much as right answers so if you do bite on a wrong answer you know what pitfalls to look out for come test time. Just my personal experience.

 

[Futureortho24]

Why cant you just do M1V1=M2V2??

(3M)(XmL)=(4M)(30mL)
3X=120
X=40mL

Can you not use this to solve for a neutralization reaction???

 

[pmoney]

Why cant you just do M1V1=M2V2??

(3M)(XmL)=(4M)(30mL)
3X=120
X=40mL

Can you not use this to solve for a neutralization reaction???

You could if you made that 3M into a 6. The H2SO4 releases 2 hydrogens to participate in the neutralization reaction, not just one.

 

[Futureortho24]

I think I am not understanding the Normality thing and why hydrogens matter for this? Im guessing that b/c it is a neutralization reaction, you have to find normality of each reactant first, then use N1V1=N2V2? Can someone please explain this to me!

THanks!!!!

 

[beastly115]

There was a thread about a week or two ago that explained this exact type of question with normality. Try searching "normality".

 

[grapeflavorsoda]

I think I am not understanding the Normality thing and why hydrogens matter for this? Im guessing that b/c it is a neutralization reaction, you have to find normality of each reactant first, then use N1V1=N2V2? Can someone please explain this to me!

THanks!!!!

simply put, normality is an effective concentration of the specie of interest.

for example:

1.0M of H2SO4 has 2 normal of proton and 1 normal of sulfate anion.

2.0M of HCl has 2 normal of proton and 2 normal of chloride anion.

1.0 M of H3PO4 has 3 normal of proton and 1 normal of phosphate anion.

and so on.

The acid-base reaction is reaction between two molecules. Neutralization is achieved when all basic species have reacted with equal number of acidic species. Since actual number of any chemical species is so large in typical physical sample size, we use bulk unit called mole. So in oder for x moles of acidic species to be neutralized we need exactly x moles of basic species.

so if you have 100 ml of 1.0M H2SO4, the solution has 0.1 moles of H2SO4.

But since each sulfuric acid molecule contributes 2 molecules of proton(or you be fancy and say that sulfuric has 2 normal of proton), the effective number of proton this solution will have is 0.2 moles.

Thus you will need 0.2 moles of basic species(such as sodium hydroxide, for example).

Let's say you have only 2.0 M solution of Ca(OH)2. How much of this solution is required to neutralize above acidic solution?

obviously you need some quantity of 2.0M Ca(OH)2 to give 0.2 moles of hydroxide anion.

since we dont know this quantity, let's call this quantity "Z."

so if we paraphrase above statement in mathematical language, it will be like

Z(2.0M)(2) = 0.2 mole. the number 2 accounts for the fact that each 1.0M of solution calcium hydroxide contributes to 2 moles of hydroxide(or you can be fancy and say that 1.0 M of calcium hydroxide has 2 normal of hydroxide).

obivously Z = 0.05 Liter = 50 mL.

 

[catytweety]

http://forums.studentdoctor.net/showthread.php?t=212412&highlight=normality

check this out! This was a past thread that helped me out.
Good luck!