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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: chelle on August 25, 2016, 04:29:53 AM

Title: Calculate osmolarity of ammonium acetate
Post by: chelle on August 25, 2016, 04:29:53 AM

Hi,

I have a solution of 150 mM of ammonium acetate in water. How do I calculate the osmolarity of this solution?

I understand that an osmole (Osmol) is 1 mol of particles that contribute to the osmotic pressure of a solution and that NaCl breaks into two particles in solution (both Na and Cl), thus giving 2 Osmols/liter. I just don't know how to do this for ammonium acetate!

From my reading online it seems ammonium acetate will disassociate into CH3COO- (aq) + NH4+ (aq), giving 2 particles in solution and thus be 2 Osmols. If this is the case the osmolarity would be 300. Is this correct?

Any help would be most welcome!

Title: Re: Calculate osmolarity of ammonium acetate
Post by: AdiDex on August 29, 2016, 03:40:56 AM

Caution : In case of NaCl , there is one assumption associated that NaCl dissociates 100% into Na+ and Cl- .

Do you think Ammonium Acetate will completely dissociated ?? (Figure out Why NaCl dissociated 100% )

Do you know about dissociation constant  ?? What is the dissociation constant of Ammonium Acetate ??

Title: Re: Calculate osmolarity of ammonium acetate
Post by: AWK on August 29, 2016, 04:13:23 AM

Quote

Is this correct?

It is  a very good approximation.

Title: Re: Calculate osmolarity of ammonium acetate
Post by: AdiDex on August 29, 2016, 04:40:03 AM

Quote from: AWK on August 29, 2016, 04:13:23 AM

Quote

Is this correct?

It is  a very good approximation.

Sorry I read something wrong .

Title: Re: Calculate osmolarity of ammonium acetate
Post by: mjc123 on August 30, 2016, 04:56:32 AM

Whether the ammonium acetate completely ionises or not, there will be two moles of species in solution, because there are two on either side of the equation:
NH₄OAc(s)  :rarrow: NH₄⁺(aq) + AcO^-(aq)  ::equil:: NH₃(aq) + HOAc(aq)
There are no NH₄OAc "molecules" in solution. Of course there will be a tendency for NH₃ to come out into the gas phase (you can smell it) but to a first approximation you will get 2 moles of dissolved species per mole of NH₄OAc. The osmolarity will of course be 0.300, not 300.

Title: Re: Calculate osmolarity of ammonium acetate
Post by: AWK on August 30, 2016, 05:04:30 AM

Quote

Of course there will be a tendency for NH3 to come out into the gas phase (you can smell it)

Smell of acetic acid is much more intense!