[CHM 112] Borax Lab Calculations

[Andrew J. Pounds]

>/Dr. Pounds, />//>/I had a question on the calculation of the concentration of [B4O5(OH)4
2-] and [Na]. />//>/I understand that we need to use the volume of HCl used and it's
molarity, so would it be />//>/M1V1=M2V2 />/V2= total volume after titration />/M2 = concentration of borax />/M1= molarity of HCl />/V1= volume of HCl />//>/And would [Na] be equal to that of borax? />//>/Thank you, /

Let's make sure that we get this right. It will really help if you look
at the first page of the lab procedures when going through this.

For each of the B_4 O_5 (OH)_4 ^2- you need to use 2 H^+ .  So the
concentration of the B_4 O_5 (OH)_4 ^2- is going to be 1/2 the moles of
acid used in the titration divided by the original volume you collected
in the test tube (5 ml).  Now, the K_sp expression is...

K_sp = [Na^+ ]^2 [B_4 O_5 (OH)_4 ^2- ] = (2x)^2 (x)

The concentration of the B_4 O_5 (OH)_4 ^2- is going to be "x".  The
concentration of the Na^+ is going to be "2x", and the value of  K_sp =
4x^3 .

Got it?


Then prepare a plot of ln(K_sp ) vs. 1/T.  Be sure to use Kelvin for the
temperature.   From the equation

$\ln(K) = (−\Delta H^{\circ}/R)(1/T) + \Delta S^{\circ}/R$

You can determine the value of $\Delta H^{\circ}$ from the slope and
$\Delta S^{\circ}$ from the y-intercept.


-- 
Andrew J. Pounds, Ph.D.  (pounds_aj at mercer.edu)
Professor of Chemistry and Computer Science
Director of the Computational Science Program
Mercer University,  Macon, GA 31207   (478) 301-5627

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