# How do you calculate the number of moles?

Jun 25, 2014

A mole can be defined as the amount of substance. It can be expressed as grams, liters, atoms, molecules, or particles.

In grams, a mole is one formula mass.

For example, 1 mol of sodium (Na) has a mass of 22.9898 g (the mass on the periodic table). And 1 mol of chlorine (Cl) has a mass of 35.453 g (the mass on the periodic table).

If you had the compound sodium chloride (NaCl), you would add the mass of 1 mol of sodium and 1 mol of chlorine. This would give you the mass of 1 mol of sodium chloride which would be about 58.443 g.

If your compound were potassium sulfide (K₂S), you would add the mass of 2 mol of potassium (2× 39.097 g) and the mass of 1 mol of sulfur (32.064 g). This would be the mass of 1 mol of K₂S.

To convert between grams and moles, you would use the substance's molar mass. To go from grams to moles, divide the grams by the molar mass.

For example, if you have 600 g of NaCl, then

$\text{600 g"/"58.443 g/mol}$ = 10.27 mol of NaCl.

To go from moles to grams, multiply by the formula mass. For example if you have 17.0 mol of NaCl, then

$\text{17.0 mol" × "58.443 g"/"1 mol}$ = 994 g

It has been found that 1 mol of any gas at STP (Standard Temperature and Pressure = 0 °C and 1 atm) occupies 22.4 L.

So, to convert between liters and moles you would use the conversion factors

$\text{22.4 L"/"1 mol}$ and $\text{1 mol"/"22.4 L}$

For example if you had 6.00 mol of carbon monoxide (CO) at STP, then

$\text{6.00 mol" × "22.4 L"/"1 mol}$ = 134.4 L of CO.

If you had 350 L of CO at STP, then you would have

$\text{350 L" × "1 mol"/"22.4 L}$ = 15.6 mol of CO

Lastly, it has been calculated that one mole of any substance contains 6.02 × 10^23 atoms (if you are talking about pure elements), molecules (if you are talking about covalent compounds), or the general term, particles (used primarily for inorganic compounds).

Just as with the previous two examples, you would use this number, 6.02 × 10^23 (Avogadro's number) to convert from particles, atoms, or molecules to moles.

Whenever you go to the mole, divide by Avogadro's number. When you go to the unit from moles, multiply by Avogadro's number.

I hope this helped without getting too confusing.