converting from equivalents to grams

[SaintJude]

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I just had a passage-based question from a Kaplan FL that required me to convert from milliequivalents (mEq) to grams.

I guess there was an AAMC 9 question that inclued mEq so Kap created a question on it. But I was wondering: Is this something that any other test preps emphasize?

Ps. mEq = mg x Valence / (molecular weight)

1. What is the meaning of an equivalent?
2. Say it takes 0.2 meq of base to increase 10 grams of soil from a pH to 5 to 6. Then why does it take it takes 0.4 meq to increase it 20 kilograms?

I see the math, but I don't understand conceptually, how you can set up a direct ratio between the two scenarios.

 

[Class1P]

an Equivalent, I looked up, is 1 mole of H+ or OH-

0.2meq would be 0.0002 moles of OH- ions

 

[chiddler]

edit: Wikipedia disagrees with me. =/

If you have 0.5 mol of HCl, an equivalent of NaOH would be 0.5 mol. Two equivalents is 1 mol.

If you have 0.3 mol of X, an equivalent of Y is 0.3 mol.

etc.

It is an amount relative to the rest of the reaction that is being looked at. It can be one mol, but not necessarily.

i don't understand your second question: "Then why does it take it takes 0.4 meq to increase it 20 kilograms" :?

 

[SaintJude]

Yes, ok, but I didn't realize that it's almost proportional. So if you want to dilute 20 grams vs 10 grams, then you have to use double the base. Ok, when I wrote that out, I guess that makes sense.

 

[chiddler]

an Equivalent, I looked up, is 1 mole of H+ or OH-

0.2meq would be 0.0002 moles of OH- ions

are you sure this is right?

 

[SaintJude]

I mean in general. Like what is the "equivalent weight" of a compound?