what is a theoretical freezing point?

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ΔTF = The change in the freezing point.
ΔTF = KF · b · i = Molality * (-1.858)(2) = Molality * (-3.716)
Molality = moles salt/Kg water

"If the solution is treated as an ideal solution, the extent of freezing point depression depends only on the solute concentration that can be estimated by a simple linear relationship with the cryoscopic constant:

ΔTF = KF · b · i

ΔTF, the freezing point depression, is defined as TF (pure solvent) - TF (solution).
KF, the cryoscopic constant, which is dependent on the properties of the solvent, not the solute. Note: When conducting experiments, a higher KF value makes it easier to observe larger drops in the freezing point. For water, KF = 1.853 K·kg/mol.[4]
b is the molality (mol solute per kg of solvent)
i is the van 't Hoff factor (number of ion particles per individual molecule of solute, e.g. i = 2 for NaCl, 3 for BaCl2).

This simple relation doesn't include the nature of the solute, so this is only effective in a diluted solution. For a more accurate calculation at a higher concentration, Ge and Wang (2010)[5] [6] proposed a new equation:

ΔT_F =ΔH^{fus}_{T_F}-2RT_{F}{\cdot}ln(a_{liq})-\sqrt{2ΔC^{fus}_{p}T^{2}_{F}Rln(a_{liq})+(ΔH^{fus}_{T_F})^2}}{2(ΔH^{fus}_{T_F}{T_F}+ΔC^{fus}_p/2 - Rln(a_{liq}))

In the above equation, TF is the normal freezing point of the pure solvent (0oC for water for example); aliq is the activity of the solution (water activity for aqueous solution); ΔHfusTF is the enthalpy change of fusion of the pure solvent at TF, which is 333.6 J/g for water at 0oC; ΔCfusp is the differences of heat capacity between the liquid and solid phases at TF, which is 2.11 J/g/K for water.

The solvent activity can be calculated from Pitzer model or modified TCPC model, which typically requires 3 adjustable parameters. For the TCPC model, these parameters are available at reference [7] [8] [9] [10] for many single salts." -- http://en.wikipedia.org/wiki/Freezing-point_depression#Calculation

I suspect that you are actually using a solution of Acetic Acid, and the actual solvent is water. Acetic Acid is one of the solutes.
Enthalpy of Fusion = 6.01 kJ/mol http://en.wikipedia.org/wiki/Water_%28data_page%29#Thermodynamic_properties
Entropy of Fusion = 22.0 J/(mol·K)
Molal Freezing point Constant = −1.858 °C kg/mol
If your salt is NaCL, j = 2...Show more