[ScottC]

I need a little help here, leaving for NY tomorrow morning and don't have the time to look into these 2 problems:

What is the equivalent weight of H3AsO4 (with calculations )

When is the molecular weight of an acid the same as its equivalent weight?

Thanks a lot!!!

Scott

[PerryinCA]

Well, I'll admit that my focus has strayed from chemistry....but I don't understand what you are looking for. I am guessing this is in terms of aqueous equilibria, in which case you need to know the base that your trying to find the equivalence point for, and how much, M...ect. or maybe I am just overthinking the question.....

-Perry

[Biomanjcs72]

Perry, I'm still trying to figure out what someone would want with
Hydrogen Arsenate anyway????? (don't think it has an Acid name persay), but that's the name of what he put up there....

[popcorn]

Couldn't find a reference to this chemical (although I did find a lot of others...) Sounds like something a chem professor would dream up. (So it this for physical chem, or analytical chem? Or some really creative inorganic chem prof? )

Arsenic and its varied incarnations have a colorful history. They also tend to do interesting things to the GI tract. And the rest of the body, for that matter.

Acid-Base equilibria was never my thing... I can do bicarbonate, but that's about it. (As that's all I use now.)

Good luck

[Rin]

Arsenic Acid (H3AsO4) is generally what is left over when someone is poisoned with Arsenic. The Marsh Test for detecting Arsenic is the following:

Zinc is combined into a solution of aqueous Arsenic and hydochloric acid to give AsH3 (g) (Arsine--very deadly) The arsine gas is passed through a heated tube where the gas breaks down into As (s) and H2 (g). The gas forms a shiny film on the inside of the tube. As little as 0.5 milligrams of As can be detected this way.

As for equivelent weight, here is my guess (I could not find any direct references to it in any of my books):

eq wt = (Molarity of solution * volume) * molar wt

eq wt = molar wt when Molarity of solution * volume = 1

Keep in mind this is just a guess.

Good luck

[ScottC]

The questions are from my lab book...we are determining the adid content of household vinegar (Normality and titration)

The end of the experiment poses a few questions based on the principal of the lab, just with different substances.

1. What is the equivalent weight of H3AsO4?
2. When is the molecular weight of an acid the same as its equivalent weight?
3. Why does it matter if water is added to the vinegar sample AFTER it has been measured?

etc..

thanks!!
Scott

[wgscott]

Originally posted by ScottC
I need a little help here, leaving for NY tomorrow morning and don't have the time to look into these 2 problems:

What is the equivalent weight of H3AsO4 (with calculations )

When is the molecular weight of an acid the same as its equivalent weight?

Thanks a lot!!!

Scott

Ok. look at your periodic table.

oxygen = 15.9994
As=74.9216
H= 1.0079

So 3X1 + 1 X 74.9 + 4 x 16 = 142.9

That is the molecular weight. The equivalent weight is this divided by the number of protons the thing liberates, which is 3. Hence the equivalent weight is 47.64.

[wgscott]

The equivalent weight is equal to the molecular weight iff the acid is monoprotic, or has only one proton per molecule that ionizes.

Vinegar? Acetic acid is CH3-COOH. I hope you weren't asking about that instead of arsenic acid. Anyway, that has one ionizable proton.

The more dilute an acid, the closer to neutrality the pH of the solution. But it shouldn't matter if you you add water after you quantify the amount of acid. I can only guess at the experiment...

[ScottC]

Thank you...exacty the info I needed...I knew the molecular weight, just unsure about how to shape it into the equivalent weight.
thanks thanks thanks!!!
Happy Thanksgiving all!!!

Scott