How to Find the Relative Molecular Mass of a Substance

The relative atomic mass is the ratio of an atom mass to 1/12 of the mass of an atom of carbon. Atomic relative masses are given in the periodic table of the elements. Any substance consists of atoms, and the relative molecular mass can be found from the respective atomic masses. As an example, we will calculate the relative molecular mass of the organic compound C4H9O2Na, which is the sodium salt of butyric acid.

Things You'll Need

• Calculator
• Periodic table of the elements

Instructions

1. Use the periodic table to obtain the relative atomic mass of each of the elements comprising the substance. In our example, the elements carbon (C), hydrogen (H), oxygen (O) and sodium (Na) have atomic masses of 12, 1, 16 and 23, respectively.

2. Multiply the atomic mass by the number of atoms in the substance for each element in Step 1. In our example, for carbon we have 12 x 4 = 48; for hydrogen, 1 x 9 = 9; for oxygen, 16 x 2 = 32; and for sodium, 23 x 1 = 23.

3. Sum the values obtained in Step 2 to calculate the relative molecular mass. In the example, the molecular mass of the compound C4H9OCl is 48 + 9 + 32 +23 = 112.

References

• "Chemistry," K.W. Whitten, R.E. Davis, L. Peck, and G.G. Stanley, Brooks Cole, Feb. 17, 2009.

Resources

• Periodic Table of the Elements