MadSci Network: Chemistry

Subject: Concentration of an Ammonium Formate solution

Date: Thu Oct 5 10:41:53 2000
Posted by James Coleman
Grade level: No grade entered. School: No school entered.
City: No city entered. State/Province: No state entered. Country: No country entered.
Area of science: Chemistry
ID: 970756913.Ch

For a HPLC mobile phase prep, ammonia and formic acid are combined in water
to form an ammonium formate solution.  The "recipe" calls for 2.0 mL of
ammonia and 2.5 mL of formic acid, diluted to a final volume of 2000 mL.
The resultant solution is being called a 0.1% Ammonium formate solution. My
instinct says, "Why isn't it a 0.225 % solution?".  Then I'm reminded of
the limiting reagent principle, and told that since the concentration of
ammonia is 0.1%, that's what the final concentration is. But don't we need
to think in terms of moles rather than percents? Starting with the
assumptions (always dangerous, but let's do it anyway, for simplicity),of
100% purity of the reagents, and 1 mL volume = 1 g weight:

2 g of ammonia @ 46 g/mole = .043 mole;
2.5 g of formic acid @ 17 g/mole = .15 g/mole;
Ammonia is the limiting reagent, and .043 moles of ammonium formate are
.043 moles of ammonium formate @ 63 g/mole = 2.7 g of ammonium formate;
Calculating the final concentration, 2.7 g / 2000 mL = 0.14%
I suppose 0.14% is close enough to 0.1% for general purposes, but I don't
think it would get us a very high score on our chemistry SAT's.  Correcting
for densities might get us a little closer, maybe not - but do I have the
general concepts correct?  Looking forward to your enlightening reply.
Thank you.

Re: Concentration of an Ammonium Formate solution

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