Even my teachers dont know this one???

[lab-woman]

Hi everyone,

I was just wondering if you can answer this chemistry question for me?

I need to make 2M Ammonium Molybdate and only 250mls of it (for a senior chem class).

If the Molecular weight on the bottle is 1235.86, I know i have to times that by 2 for 2M and then divide total by 4 for a 250ml solution.

The answer is 617.93g of powder. Ok...that is a lot of powder.

How much water do i add...... 250mls???? Then its not going to be a 250ml solution is it? Its going to be around 800ml i am guessing because of the huge amount of powder.

But its supposed to be 250mls?

Any thoughts.....my teachers are all stumped.

[fibreweb]

2M Ammonium Molybdate is impossible to make.
The solubility of Ammonium Molybdate is approximately 50gm/100mls
125 gms at the most will dissolve in 250mls.
What does she need it for?
The 0.1 M solution I have in the Chem Store is not all dissolved.
For the record, to make up 250 mls of a solution I would use a 250ml volumetric flask.
Using the amount of chemical required to make 250 mls and then distilled water up to the line.

[macca]

What are they using it for? We don't ever use above 0.5m

[smeee]

Its for Yr 12 Chemistry - Qualitative tests for assessing water quality. I haven't got any so teacher will have to fudge it !
Not sure if we can use any other "molybdate"
The prac from conquering Chemistry doesn't give a molarity.

[ELIZABETH]

We use 0.1M ammonium molybdate for that....works OK.