How to Calculate Molecules, Elements, and Atoms

How to calculate Molecules, Elements, and Atoms

Parts of a chemical formula: Let’s look a the following formula to understand the different parts. The part that is underlined will match the term that is given.

Coefficient (number in front): 3 H2O Subscript: 3 H2O (The number 2 is underlined) Compound with Element Symbols: 3 H2O

(Hint – In chemistry, when there is no number present, either for a coefficient or subscript, it is implied that there is a 1.)

Molecules – This is by far the easiest part to calculate. In order to do this, look at the coefficient (number in front). This number tells you the number of molecules that you have.

Elements – This is deceptively easy. Most of the time you can simply count the number of capital letters to determine how many there are. For example:

3 H2O – There are two capital letters (H and O). This means that 2 different elements are present (Hydrogen and Oxygen)

Careful!!! – Look at the following:

C6H5OH – In this example there are only 3 different elements. Hydrogen is in the formula twice. If we only counted capital letters we would have mistakenly counted four elements.

Atoms – This involves some mathematical calculation to get the total number of atoms. Lets look at some basic principles to solve the following problems (remember: no number implies a 1).

H2O – The subscript refers to the number of atoms for the element that it follows. In this example there are 2 Hydrogen atoms and 1 Oxygen atom. This gives us a total of 3 atoms.

3 H2O – This is the same molecule (water), but we have placed a coefficient of 3 in the front. This coefficient tells us that we have 3 water molecules. Each molecule has 2 Hydrogen atoms and 1 Oxygen atom. So, at this point we will use our coefficient to multiply the number of atoms in H2O by 3. This will give us a total of 9 atoms.

Go on to the next page… 5 Ca(O3H)2 – When parentheses are involved, don’t panic. What this means is that the subscript of 2 multiplies everything in the parenthesis, but only in the parenthesis. If we ignore the coefficient at the front (the 5) for a moment, then we have 6 Oxygen atoms and 2 Hydrogen atoms. The calcium is unchanged by the subscript of 2, so we have only 1 Calcium atom. Now let’s bring back the coefficient and multiply everything by 5. This now means that there are 5 Calcium atoms, 30 Oxygen atoms, and 10 Hydrogen atoms. This gives us a total of 45 atoms.

Like anything that you are going to be good at, this will take some practice, so be sure that you are studying every night.