Chemical Forums

Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: FAC208aussie on September 10, 2009, 12:30:19 AM

Title: Mass Balance Equations
Post by: FAC208aussie on September 10, 2009, 12:30:19 AM
I am having trouble comprehending the mass-balance eqn.
.25 M in NaF and saturated with CaF2
Would my eqn then be:

.25 M = Na+ = F- + CaF2

both of those are in concentraitons.
Title: Re: Mass Balance Equations
Post by: poca on October 01, 2011, 09:26:48 PM
*Ignore me, I am impatient*, I am working on this exact problem

If CaF2, is not soluble how do you include it in the equation?
Title: Re: Mass Balance Equations
Post by: Borek on October 02, 2011, 05:44:57 AM
Please elaborate on what the question is, as it looks ambiguous.

Mass balance of what? There are several mass balance equations for this system, some are not important, at least one is necessary to find the equilibrium.
Title: Re: Mass Balance Equations
Post by: poca on October 02, 2011, 08:09:09 AM
The question is, Write the mass balance expressions for a solution that is.
.25M NaF and saturated with CaF2

.25M = [Na+]
.25M = [F-]+

I should mention I'm very lost on how to do these problems...
Title: Re: Mass Balance Equations
Post by: Borek on October 02, 2011, 01:53:15 PM
OK.

You are right about sodium, however, you are wrong about fluorides. Where do they come from?

Title: Re: Mass Balance Equations
Post by: poca on October 02, 2011, 04:32:48 PM
.25M F from NaF, but CaF2 is solid, i guess if it dissolves then excess of CaF2, how much does that contribute
.25M= [F-] + [F-]2
Title: Re: Mass Balance Equations
Post by: Borek on October 02, 2011, 05:02:10 PM
You never square anything in mass balances, you just use stoichiometric coefficients.

There are two sources of fluorides - sodium fluoride and dissolved calcium fluoride. Can you express concentration of fluorides in terms of concentrations of sodium and calcium?
Title: Re: Mass Balance Equations
Post by: poca on October 02, 2011, 05:33:19 PM
[Na] = [F] = .25M
2[Ca] = [F] = ?
Title: Re: Mass Balance Equations
Post by: poca on October 02, 2011, 09:36:28 PM
[F-] = [Na] + 2[Ca]
Title: Re: Mass Balance Equations
Post by: Borek on October 03, 2011, 05:08:55 AM
Quote from: poca on October 02, 2011, 09:36:28 PM
[F-] = [Na] + 2[Ca]

And that's correct.

You already wrote another one earlier - describing concentration of Na+.
Title: Re: Mass Balance Equations
Post by: luke1987 on November 15, 2012, 03:31:41 PM
Why wouldn't FAC's original equation be correct? I don't understand why you would set concentrations equal to [F-], since there are two sources...wouldn't you set equal to [Na+] because there is only one source?