|MadSci Network: Chemistry|
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 formed; .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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