Phase Equilibria Datum Dialog
Applicability: Cranium, Synapse (core versions 0315+)

When compositions are entered in molar units, additional information is needed to convert these values into the common composition units used by Cranium and Synapse, i.e., mol frac, mol %, wt frac, wt %. (For general reference, we highly encourage researchers to report all their compositions in terms of weight fraction or weight percent. These units are typically closely tied to actual experimental measurements and need no further interpretation.)

A composition value given in molar units, Mi, equals the number of moles of component i in a one liter solution. The solution is mixture of solutes and solvent. Thus, the weight fraction of component i is given by (where Mwi is the molecular weight of component i and ρs is the solution's density):

For example, a 0.35 molar solution of sodium acetate (Mw = 82.034) in water (solution density = 1.01203 at 25 C) is converted to a weight fraction of 0.0284:

However, in this previous calculation, we assumed sodium acetate was the solute and water was the solvent. Although this choice may seem obvious, the designation of solute and solvent becomes more arbitrary if we have a 0.35 molar solution of sodium acetate in a mixture of 50% by weight ethanol and water.

Thus, to be certain, Cranium and Synapse both require you to specify the solvent when entering molar concentrations.

The previous calculation also shows that you must enter the solution's liquid density. Since liquid density is a temperature dependent property, it is important to enter the density at the temperature at which the molarity was recorded.

Entering a Molar Concentration

All Cranium and Synapse dialog's used to enter composition contain a Molar Solvent and Solution Density controls.

Selecting 'molar' for the composition units enables the Molarity Solvent control and the Solution Density control. These controls enables you to select which component should be designated as the solvent and enter the solution's liquid density at the temperature of the molarity measurement. By default, the last component of a mixture, e.g., n-octane in the image above, is designated as the solvent.

When molar units are selected, you can enter molar amounts for each of the solutes. The composition control for the component designated as the solvent is not enabled.

Converting a Molar Concentration

The mixture composition shown in the image to the right consists of 2.243 moles of n-butane and 2.269 moles of n-hexane in solution with n-octane. The liquid density of this three component mixture is 651.86 kg/m3.

Using each component's molecular weight, we can calculate the mass of each solute.

Component Molarity Mw Mass [g]
n-Butane 2.243 58.123 130.370
n-Hexane 2.269 86.177 195.536
Total - - - - - - 325.906

One liter of solution has a mass of 651.86 g. Subtracting the masses of n-butane and n-hexane from this amount gives us a remainder of 325.954 g, which is the mass of n-octane.

Dividing each of the mass values by the mass of one liter of solution gives us the following weight fractions:

Component Calculation Wt Frac
n-Butane 130.370 / 651.86 0.20
n-Hexane 195.536 / 651.86 0.30
n-Octane 325.954 / 651.86 0.50
Related Documentation
Topic Description
Getting Started using Synapse provides a quick tour of Synapse's capabilities including examples of chemical product design.
Getting Started using Cranium provides a quick tour of Cranium's capabilities including a discussion of structure editing.
Estimating Chemical Properties a short video demonstrating how to estimate the physical properties of chemicals using either Synapse or Cranium.
Estimating Mixture Properties a short video demonstrating how to estimate the physical properties of mixtures using either Synapse or Cranium.