By Riti Gupta
Updated Aug 30, 2022

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In the laboratory, chemists routinely work with quantities that are either astronomically large or minutely small. While a clear salt solution is easy to see, the individual molecules it contains are far beyond the reach of the naked eye.

So, how do we determine the exact number of salt molecules in a given volume? The answer lies in a fundamental constant known as Avogadro’s number.

What Is Avogadro’s Number?

Within a salt solution, the molecules are invisible and countless—often exceeding 10²³ in a single mole. Chemistry relies on knowing how many particles are present to predict reactions, prepare solutions, and maintain precision in measurements.

The concept of the mole bridges the gap between these unfathomable quantities and everyday laboratory work. One mole of any substance contains exactly 6.022 × 10²³ particles—this value is known as Avogadro’s number.

Think of a mole like a dozen: just as a dozen donuts always equals twelve donuts, a mole of sodium chloride always contains 6.022 × 10²³ NaCl molecules.

The relationship between moles and mass is expressed through the molar mass, the number of grams in one mole of a substance. The molar mass for each element is listed beside its symbol on the periodic table. For instance, carbon’s molar mass is 12.01 g/mol, meaning one mole of carbon weighs 12.01 grams.

Using Avogadro’s Number to Calculate the Number of Molecules

Suppose you have 2 moles of NaCl. To find how many molecules that represents, multiply by Avogadro’s number:

\(\mathrm{2\,mol\,NaCl\Bigl(\frac{6.022\times10^{23}\,\text{molecules}\,NaCl}{1\,mol}\Bigr)=1.2\times10^{24}\,\text{molecules}\,NaCl}\)

Thus, 2 moles of NaCl contain 1.2 × 10²⁴ molecules.

What if you’re given 2 grams of NaCl instead of moles? First, convert grams to moles using the molar mass (58.44 g/mol for NaCl), then apply Avogadro’s number:

\(\mathrm{2\,g\,NaCl\Bigl(\frac{1\,mol\,NaCl}{58.44\,g\,NaCl}\Bigr)\Bigl(\frac{6.022\times10^{23}\,\text{molecules}\,NaCl}{1\,mol}\Bigr)=2.1\times10^{22}\,\text{molecules}\,NaCl}\)

So, 2 grams of NaCl contain approximately 2.1 × 10²² molecules.