When determining the formula of a compound, you must know the mass of each element that makes up the compound. You must also know how many atoms of each element make up the molecule of the compound.
To find the formula of a compound, you must combine the masses of each element with the correct number of atoms. Once you do this, you will have determined the formula of the compound.
In this blog post, we will discuss how to find the molecular formula of a substance using iodine as an example. Iodine is a nonmetallic gray element that is part of many compounds, including table salt.
Table salt (NaCl) is a molecule made up of one atom of sodium and one atom of chlorine. Sodium chloride is its chemical name. It is an ionic compound, which means that there are ions within it.
Use the mass ratios to determine the elements
Now that you know the mass of one atom of each element in the compound, you can use that information to determine the other elements. You can do this by determining the mass ratio of each element.
For example, if there are 132 amu of sodium per 100 amu of chlorine then sodium makes up 1/6 (100/132) of the compound. Chlorine, therefore, makes up 5/6 (100/132) of the compound.
This is an important step in figuring out the molecular formula because it tells you how many atoms of each element are in the molecule. Once you know that, you can determine the formula!
At this point, you could also determine the molecular mass of the compound by weighing it and dividing that number by Avogadro’s number (6.02 × 1023 amu). This would give you a more accurate molecular mass than what was given in the original question.
Use the periods to determine the elements
The first step to determining the molecular formula is to identify the element symbols. There are seven: carbon (C), hydrogen (H), oxygen (O), nitrogen (N), chlorine (Cl), fluorine (F), and iodine (I).
Here, we can already tell that there are three atoms of each element, so we do not have to worry about double counting them. How do we know?
Well, first of all, it is clear that there are three atoms of each element based on the periodic table. Second of all, you can look at the amu count and see that it is 132 amu total, which means there must be three atoms per molecule.
The only thing left to do is determine the molecular mass of the compound. The molecular mass is how much one molecule weighs.
Use the groups to determine the elements
Now that you have the total number of atoms, you can determine the elements that make up the compound. To do this, you need to know the different chemical groups that exist.
Each group has a specific number of atoms or pairs of atoms that it contributes to the molecule. By identifying these groups, you can determine what elements make up the compound.
For example, in this case, we have a carboxylic acid group (-COOH) and a benzene ring (-CH), so our compound is most likely made up of hydrogen, oxygen, and carbon.
Other possible elements that could be part of the compound are nitrogen and chlorine. You would have to check if those were present by looking at the Lewis structure for each element.
Compare isotopes to determine values
Once you have the mass of all atoms in a molecule, you can determine the molecular formula. You can do this by comparing the masses of the atoms to standard values.
For example, hydrogen is compared to the standard mass of one hydrogen atom, which is 1 amu. Oxygen is compared to eight oxygens, which have a standard mass of 16 amu.
So if a molecule contains one hydrogen and eight oxygens, then the molecular formula is HO. This is how water gets its formula!
Iodine has several isotopes, so it requires a more in-depth comparison to determine its average atomic mass. Iodine (I) has an atomic mass of 127 amu, so if the compound contains 12 grams of Iodine per 100 grams of compound then the ratio is 12:100 or 1:8. This means that there is one iodium atom for every eight atoms in the other element(s). The molecular formula is therefore I8.
Use online tools to calculate molecular mass
Once you have determined the mass of each atom that makes up the compound, your next step is to find the molecular mass, or how many atoms are in one molecule of the substance.
There are a number of online tools that can help you calculate the molecular mass of a compound. Most of these sites require you to enter the elemental composition of the compound, which you have already done after completing steps one and two.
One site offers a more in-depth look at how to calculate molecular mass, and it also provides examples of different compounds to check your work.
Step five: Calculate the molar mass
After determining the weight of one molecule of a substance, your next task is to find how many molecules are in one liter (1L) of the material. To do this, you must divide the total weight by the weight of one molecule. This gives you the molar mass.
Calculate average atomic mass
Now that you have the mass of one atom of each element, you can calculate the average atomic mass of each element. To do this, divide the total mass by the number of atoms of each element.
For example, if the experiment produced 100 grams of compound X and it contained 50 grams of nitrogen and 50 grams of hydrogen, then the average atomic mass of nitrogen is 50g/6=8g and hydrogen is 50g/1=50g. The total average atomic mass is therefore 108g.
You can also use this method to find the average molecular mass (AM)of a compound. What is AM? It is just the average atom weight in a molecule! To find it, divide the total weight of the compound by the number of molecules it contains.
Compound X contains 100 molecules and has an AM of 132 amu (atomic masses). Thus, Compound X is composed entirely of nitrogen.
Determine if compound or molecule
The next step is to determine if the compound you have determined is a molecule or compound. If the total number of atoms in the molecule is six or less, then it is considered a carbon atom.
If the total number of atoms in the molecule is eight or more, then it is considered a molecule. A lot of times, people assume that everything is a molecule, but this isn’t always the case!
For example, water (H2O) is not considered a molecule because the total number of atoms in one water molecule is two (hydrogen and oxygen), which makes it a component of a larger unit (water).
Write out formula for compound or molecule
Now that you know the mass of one molecule of the compound, you can determine the molecular formula. You need to divide the mass of one molecule by the mass of one atom of each element in the molecule.
So in this case, you would divide 132 amu by 26 amu, which is lithium. Then you would add together all of the masses of all of the atoms in boron (10 amu) and hydrogen (1 amu).
You would then get a molecular formula of C 7 H 8 , which means there are eight carbon atoms and seven hydrogen atoms for every atom of lithium. This is not always necessary, but it can be helpful to know!
The compound is not just lithium fluoride, it is lithium fluoride octahydrate! Octahydrate refers to the water molecules that are attached to the compound.
