How to Find the Empirical Formula

Step-by-Step Guide

1. 1

   Step 1: Know what the empirical formula is.

   In chemistry, the EF is the simplest way to describe a compound—it is basically a list of the elements that make up a compound, organized by percentage.
   
   It's important to note that this simple formula does not describe the arrangement of the atoms within the compound, it simply states which elements compound is made of.For example:
   A compound that is made up of
   40.92% Carbon,
   4.58% hydrogen, and
   54.5% Oxygen would have an empirical formula of C3H4O3 (we will go through an example of how to find the EF of this compound in Part Two).
2. 2

   Step 2: Understand the term 'percent composition'.

   'Percentage composition' refers to the percent of each individual atom in the whole compound that we are looking at.
   
   To find the empirical formula of a compound, we must know the percentage composition of the compound.
   
   If you are finding the empirical formula for homework, you will most likely be given the percentages.
   
   In a chemistry lab, to find the percentage composition, the compound would be examined through some physical experiments and then quantitative analysis.
   
   Unless you are in a lab, you will not need to actually do these experiments. , A gram atom is the specific amount of an element that's weight in grams equals its atomic mass.
   
   To find a gram atom, the equation is:
   The percent of the element in the compound (%) divided by the element’s atomic mass.For example, let’s say that we have a compound that is made up of
   40.92% carbon.
   
   The atomic mass of carbon is 12 so our equation would be
   40.92 / 12 =
   3.41. , When you are working with a compound, you will have more than one gram atom to calculate.
   
   After you have found all of the gram atoms in your compound, look at all of them.
   
   To find the atomic ratio, you will have to pick out the gram atom that is the smallest out of all of the gram atoms that you calculated.
   
   You will then divide all of your gram atoms by the smallest gram atom.
   
   For example:
   Let’s say that we are working with a compound that has three gram atoms:
   1.5, 2 and
   2.5.
   
   The smallest gram atom out of those three numbers is
   1.5.
   
   So to find the atomic ratio, you must divide all of the numbers by
   1.5 and then separate them with the symbol for ratio :.
   1.5 /
   1.5 =
   1. 2 /
   1.5 =
   1.33.
   2.5 /
   1.5 =
   1.66.
   
   So your atomic ratio is 1 :
   1.33 :
   1.66. , When writing an empirical formula, you need whole numbers.
   
   This means you can’t use a number like
   1.33.
   
   After you have found your atomic ratio, you need to convert any partial numbers (again, like
   1.33) into whole numbers (like 3).
   
   To do this, you have to find a whole number that can be multiplied by each individual number in your atomic ratio to get a whole number.
   
   For example:
   Try
   2.
   
   Multiply the numbers in your atomic ratio (1,
   1.33, and
   1.66) by
   2.
   
   You get 2,
   2.66, and
   3.32.
   
   These are not whole numbers so 2 doesn’t work.
   
   Try
   3.
   
   You get 3, 4, and 5 when you multiply 1,
   1.33, and
   1.66 by
   3.
   
   Therefore, your atomic ratio of whole numbers is 3 : 4 :
   5. , The whole number ratio that we just solved actually fits in to the empirical formula.
   
   Those three whole numbers are the little numbers that hang at the foot of each letter that represents a separate element of the compound.
   
   For example, our made up empirical formula would look like:
   X3Y4Z5
3. 3

   Step 3: Be aware that you will be dealing with gram atoms.

4. 4

   Step 4: Know how to find the atomic ratio.

5. 5

   Step 5: Understand how to convert numbers of atomic ratio into whole numbers.

6. 6

   Step 6: Understand what those whole numbers mean for the empirical formula.

Detailed Guide

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