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What Data Are Needed to Calculate the Molecular Mass of the Liquid?

Properly calculating molecular mass can be a trial for any beginning chemistry student. A crucial concept for solving many stoichiometry problems, calculations of molar mass can be performed whenever a few quantities are known. Typically measured in grams per mole, molar mass is used to determine predicted reaction results, limiting reactants, and other chemistry calculations. The exact method used to determine the molecular mass of a substance or product of the reaction of chemicals depends upon the specific problem and reaction perimeters.

Simple Calculations
- For simple molecular mass calculations, a summation of the individual molecules making up a compound should be enough to help you calculate molecular mass. For example, to determine the molecular mass of carbon dioxide (CO2), add 12+16+16 for a total of 44 g/mol. Individual molecule weights can be found on any standard periodic table of chemicals, where the mass number can also be used as the molecular mass. Theoretically this approach can only accurately be used to determine the molecular mass of covalent compounds, since only covalent (and not ionic) compounds are composed of molecules.
Testing Knowledge
- Another simple molecular mass calculation problem might simply test your knowledge of the definition of molecular mass. If both the number of moles and the mass of the liquid are known, then the molar mass can be determined by using the definition: mass/number of mols=molar mass.
Combining Liquids
- If a reaction involves the combination of two liquids to form a new liquid (or solid) compound, the molecular mass of this new compound can also be determined. First you must determine a balanced equation for the entire chemical reaction. You also need to determine the limiting reactant (reactant in the smallest molecular quantity). Then, set up a stoichiometric relationship calculation using the limiting reactant's original molecular mass and the balanced equation to find out the grams of the new product produced. This knowledge, combined with the methods from the formulas presented in methods one and two should be sufficient for determining the molecular mass of the new liquid.
Unknown Liquid
- The molecular mass of an unknown liquid can also be determined if you have enough other information. For example, if the density, volume, and mass of the liquid are known at a specific temperature and pressure, the relationship Pv=nRT (where P=density, v=volume, n=mols, R=ideal gas constant, and T=temperature in Kelvin) can be used to determine the "n," the number of mols. Since molar mass is equal to mass/number of moles, dividing the newly determined number of mols (n) by the previously known mass will yield the molar mass of the substance.

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