When calculating the mass of one molecule, do the following:

1. Calculate the molar mass of the substance
2. Divide it by Avogadro's Number

Make sure you have a periodic table and a calculator handy.

Example #1 - What is the mass of one molecule of H₂O?

Step #1 - the molar mass of water is 18.015 g/mol. This was calculated by multiplying the atomic weight of hydrogen (1.008) by two and adding the result to the weight for one oxygen (15.999).

Please remember that you need the molar mass first when trying to find the mass of one molecule.

Step #2 - divide the substance's molar mass by Avogadro's Number.

18.015 grams/mol

---------------------------- = 2.992 x 10¯²³ grams

6.022 x 10²³ mol¯¹

Note that the final answer has been rounded to four significant figures (from 2.9915 - note use of rounding with five rule) and that the unit of mole cancels.

Example #2 - calculate the mass (in grams) of one molecule of CH₃COOH

The molar mass of CH₃COOH is 60.06 g/mol.

60.06 g/mol

-------------------------- = 9.973 x 10¯²³ g

6.022 x 10²³ mol¯¹

Calculate the mass (in grams) for one molecule of:

1) H₂

2) PCl₅

3) UF₆

4) F₂

5) CH₄

Answers

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