Sciencemadness Discussion Board

Eqwt , Eqs , normality

aeacfm - 27-2-2011 at 11:18

some takeaway problems make me confused too much .

simply , equivalent wt what i know is that eq = Mwt/n , n = no of euivalents

n depends on the reaction (acid base , redox , ....)

1 - suppose for Na2Co3 how can i get the no equivalents (what i know is no of equivalents = no of gram equivalents) if no reaction deatiled and a weight value given x gram ?
n = wt/eqwt , eqwt = Mwt/2*1 is that right ?

2 - calculate the concentration of NaOH if 48.19 ml of 1.75 N H2SO4 neutralizes 25ml of the base ?
the answer was by :
N1 . V1 = N2 . V2

but if the context was 48.19ml of 1.75 M H2SO4 neutralizes 25ml of the base ?.... the answer was by calculating the no of moles of the acid then calculating moles of base from ratio between acid and base in the equation !!!!

pls explain

[Edited on 27-2-2011 by aeacfm]

Chordate - 27-2-2011 at 15:49

You're given the reaction. Or at least the reactants.

Sulfuric acid is diprotic, so it can potentially neutralize two moles of a strong monoprotic base for every mole of it you place in the pot.

It gets a little more dicey when you start talking about mixtures of weak acids and bases. For instance, with HF and Ammonia. Then you need to take into account the dissociation constants of each proton in order to be able to figure out what your concentrations are. There are great tutorials for this online.


aeacfm - 27-2-2011 at 21:33

thanks a lot for your reply

Quote: Originally posted by Chordate  
You're given the reaction. Or at least the reactants.



not every time , like what i figured he may give you the weight of substance and need the gram equivalent wt ....what can i do ?

Magpie - 27-2-2011 at 22:30

from Wiki:

Equivalent weight (also known as gram equivalent) is a term which has been used in several contexts in chemistry. In its most general usage, it is the mass of one equivalent, that is the mass of a given substance which will:

* supply or react with one mole of hydrogen ions (H+) in an acid–base reaction; or
* supply or react with one mole of electrons in a redox reaction.[1]

aeacfm - 28-2-2011 at 01:43

Quote: Originally posted by aeacfm  

2 - calculate the concentration of NaOH if 48.19 ml of 1.75 N H2SO4 neutralizes 25ml of the base ?
the answer was by :
N1 . V1 = N2 . V2

but if the context was 48.19ml of 1.75 M H2SO4 neutralizes 25ml of the base ?.... the answer was by calculating the no of moles of the acid then calculating moles of base from ratio between acid and base in the equation !!!!

pls explain

]


why didnt we use the formula
M 1 .V1 = M2 .V2 in the second case

Magpie - 28-2-2011 at 08:45

Quote: Originally posted by aeacfm  

why didnt we use the formula
M 1 .V1 = M2 .V2 in the second case


This is not a general formula because, for example, 1 mole may react with 2 moles:

2NaOH + H2SO4 ---> etc

whereas N1V1 = V2N2 is perfectly general. That's the whole purpose of using equivalents and normality (N) vs moles and molarity (M).