How do you determine the number of moles in a given mass?

You determine the number of moles in a given mass by dividing the mass by the molar mass of the substance.

To calculate the number of moles in a given mass, you need to know the molar mass of the substance. The molar mass is the mass of one mole of a substance, usually expressed in grams per mole (g/mol). It can be found on the periodic table for elements and calculated for compounds by adding up the molar masses of all the elements in the compound.

Once you have the molar mass, you can calculate the number of moles by using the formula:

Number of moles = Mass (in grams) / Molar mass (in g/mol)

For example, if you have 18 grams of water (H2O), you would first calculate the molar mass of water. The molar mass of hydrogen (H) is approximately 1 g/mol and there are two hydrogen atoms in water, so that's 2 g/mol. The molar mass of oxygen (O) is approximately 16 g/mol. So, the total molar mass of water is 18 g/mol (2 g/mol for hydrogen + 16 g/mol for oxygen).

Then, you would divide the mass of the water (18 grams) by the molar mass of water (18 g/mol) to find the number of moles. In this case, it would be 1 mole.

This calculation is fundamental in chemistry because the mole allows chemists to count atoms and molecules by weighing them. It's a bridge between the atom, which is too small to see or weigh, and the macroscopic world of grams and kilograms. Understanding how to calculate the number of moles in a given mass is crucial for stoichiometry, which is the calculation of quantities in chemical reactions.