Quick Chem Q: Molarity vs Normality

[BiomajorPreDent]

Hey all, I have a quick question. This is really bothering me because its so easy. When solving gen chem questions like the two below, how do you know whether your suppose to use Normality or Molarity

You are given a volume, 1 liter, at a concentration, .01M, of an aqueous acetic acid and say its titrated with a base at concentration, .10M of Ca(OH)2(aq). And they want to know what volume of basic solution must be added to reach the equivalence point..

like compared to this problem:

What volume of 12M HCl solution is required to prepare exactly 500mL of a .60M HCl solution?

(the answer to the first one is 50mL and the second one is 25mL)

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[razblo]

Hey all, I have a quick question. This is really bothering me because its so easy. When solving gen chem questions like the two below, how do you know whether your suppose to use Normality or Molarity

You are given a volume, 1 liter, at a concentration, .01M, of an aqueous acetic acid and say its titrated with a base at concentration, .10M of Ca(OH)2(aq). And they want to know what volume of basic solution must be added to reach the equivalence point..

like compared to this problem:

What volume of 12M HCl solution is required to prepare exactly 500mL of a .60M HCl solution?

(the answer to the first one is 50mL and the second one is 25mL)

I would say it depends on the starting info. Question #1 says you're doing a titration for which I believe you have to use normality.

 

[mx41]

The question is worded to use the molarity formula
M = moles/V

0.60M = x moles/0.5L
x = 0.3 moles HCl

12.0 M HCl = 0.3moles/ x Liters

x = .025 L or 25mL

 

[RCT PC CRN]

Second one is M1V1=M2V2

500 ml x .6M = unknown x 12M
unknown = 25 ml

First one, I believe the answer is 100 ml
1 L of .01M AA - .01 mol
You need .01 mol of Strong base in this case CaOH2 for titration - and the [ ] you have is .10M so that makes it 100 ml.

Answer could be 50 ml if you have 2 L of .01M AA

If you know the Normality then your job is much easier NaVa=NbVb where N=normality, a= acid, b=base so if you know any 3 quantity you can find 4th just the way in M1V1=M2V2

Can anyone confirm my view about first one?

 

[BiomajorPreDent]

Ok the answer says:

"The best way to solve this is to apply the titration formula VaNa=VbNb

We are given the volume of acid and the concentrations of both the acid and the base, but must convert the given volume to mL and the concentrations from the given molarity to normality before applying the formula.

One liter is 1000 mL. Acetic acid is monoprotic, thus its normality is the same as its molarity, Na, thus equals 0.01N. Calcium hydroxide provides two OH groups per formula, thus its normality will be twice its molarity, Vb thus equals 2(.10) or .20N.

Vb= VaNa/Nb = 1000 * .01 / .2 = 50mL"

So what im trying to figure out is, how do you know to use normality vs just molarity?

 

[sweetcinuhstix]

I agree with what everyone says, but I can add a really good tip!

You see the acid, it is MONOPROTIC.

The base, its DIPROTIC because it is (OH)2. There will be 2 equivalents of OH, and that is why you use normality for this problem, and times the molarity by 2 for the base side.

If both acid and base were monoprotic, you would just use molarity.

 

[RCT PC CRN]

So what im trying to figure out is, how do you know to use normality vs just molarity?

Molarity is for individual substance. You can use M1V1=M2V2, when you playing with individual acid or base. Other words when you trying to figure out molarity after dilution or making the substance stronger. Only individual substance.

Normality is for reaction. It involve two substance, like one in titration. Asking to find you volume of strong base (one substance) in titration of weak acid (other substance). You can do the other way around as well, find volume of weak acid that x ml of strong base can titrate.

I hope this make sense.

PS: Thanks for clarification about the answer. Does make sense with 50 ml. My bad about 100 ml.

 

[BiomajorPreDent]

Ok great. Thats what I was thinking. I was freaking out because these are simple chem questions and I dont wanna get them wrong because of molarity vs normality.

Thanks!