Molal Concentration Units
Applicability: Cranium, Synapse (core versions 0318+)

When compositions are entered in molal 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 molal units, mi, equals the number of moles of component i in a solution with 1000 g of solvent, designated as s. Thus, the weight fraction of component i is given by (where Mwi is the molecular weight of component i):

For example, a 0.35 molal solution of sodium acetate (Mw = 82.034) in water is converted to a weight fraction of 0.0279:

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 molal 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 molal concentrations.

Entering a Molal Concentration

All Cranium and Synapse dialog's used to enter composition contain a Molal Solvent control.

Selecting 'molal' for the composition units enables the Molal Solvent control. This control enables you to select which component should be designated as the solvent. By default, the last component of a mixture, e.g., n-octane in the image above, is designated as the solvent.

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

Converting a Molal Concentration

There does not appear to be a standard method for reporting the molality of multisolute mixtures. The method used by Cranium and Synapse assumes that the molality of each solute is reported based on 1 kg of solvent. Thus, mixture composition shown in the image to the right consists of 1.1 moles of n-butane, 1.2 moles of n-hexane, and 1000 g of n-octane.

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

Component Molality Mw Mass [g]
n-Butane 1.1 58.123 63.94
n-Hexane 1.2 86.177 103.41
n-Octane - - - - - - 1000.0
Total - - - - - - 1167.35

Using the mass of each component and the total mass, Cranium and Synapse would compute the weight percents as: n-Butane 5.48%, n-Hexane 8.86%, and n-Octane 85.66%.

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.