If you have homework to find the empirical formula of a compound, but don't know how to start, fear not! The wikiHow here will help you! First, consider the basic knowledge you need to find empirical formulas, and move on to the example problems in Section 2.
Step
Method 1 of 2: Understanding the Basics
Step 1. Know about empirical formulas
In chemistry, an empirical formula is the simplest way of expressing a compound -- it's basically a list of the elements that make up a compound, arranged by percentage. You need to understand that this simple formula does not describe the "arrangement" of the atoms in the compound, this formula only states the constituent elements. For example:
A compound composed of 40.92% carbon, 4.58% hydrogen, and 54.5% oxygen will have the empirical formula C3H4O3 (we'll cover an example of how to find this empirical formula in part two).
Step 2. Understand the term 'mass percent'
Mass percent means the percent sum of every single atom in a compound. To find the empirical formula of a compound, we must know the mass percent of the compound. If you are looking for an empirical formula for a homework problem, you will most likely be given information about the mass percent.
In a chemistry laboratory, to find the percent by mass, a compound will be tested by several physical tests and then analyzed quantitatively. Unless you're in a lab, you don't really need to do this test
Step 3. Be careful when dealing with gram atoms
A gram atom is the specific amount of an element whose weight in grams is equal to its atomic mass. To find the gram atom, the equation is: The percentage of an element in a compound (%) divided by the atomic mass of that element.
For example, let's say we have a compound that is made up of 40.92% carbon. The atomic mass of carbon is 12, so our equation will be 40.92/12 = 3.41
Step 4. Know how to find the atomic ratio
When you solve a compound problem, there will be more than a gram of atoms that you have to count. Once you have all the grams of atoms in your compound, pay attention. To find the atomic ratio, you must choose the atomic gram that is the smallest of all. You must then divide all of your atomic grams by the smallest atomic gram. For example:
- Let's say we are working on a compound that has three grams of atoms, namely: 1, 5; 2; and 2. :
- 1,5 / 1, 5 = 1.2 / 1, 5 = 1, 33. 2, 5 / 1, 5 = 1.66. So the atomic ratio is 1: 1, 33: 1, 66.
Step 5. Understand how to convert atomic ratio numbers to integers
When writing empirical formulas, you must use whole numbers. This means you can't use numbers like 1.33. Once you find your atomic ratio, you'll need to convert a fractional number (such as 1.33) to an integer (such as 3). To do this, you must find a number that can be multiplied by each number in your atomic ratio to get an integer. For example:
- Try 2. Multiply the number in your atomic ratio (1, 1, 33, and 1, 66) by 2. You get 2, 2, 66, and 3, 32. These numbers are not integers, so you can't using a multiplier of 2.
- Try 3. You'll get 3, 4, and 5 when you multiply 1, 1, 33, and 1, 66 by 3. Thus, the atomic ratio of integers is 3: 4: 5.
Step 6. Understand that these integers are used for empirical formulas
The integer ratios we just solved can be used in empirical formulas. The three integers are the numbers placed at the foot of each letter that represent the different elements in a compound. For example, the empirical formula for the compound we are making is:
X3Y4Z5
Method 2 of 2: Finding Empirical Formulas
Step 1. Determine the mass percent of your compound
If you try to find an empirical formula for a homework problem, you'll most likely be given mass percent data -- you just need to know where to find it. As an example:
- Let's say that the sample problem asks you to look at a sample of vitamin C. It says carbon 40.92%, hydrogen 4.58% and oxygen 54.5% -- this is mass percent.
- 40, 92% of vitamin C is composed of carbon, while the rest is composed of hydrogen as much as 4.58% and oxygen 54.5%.
Step 2. Find the number of grams of atoms in the compound
As explained in section 1, the equation for finding the number of grams of atoms is: The percentage of the element in the compound (%) divided by the atomic mass of the element.
In our example, the atomic mass of carbon is 12, hydrogen is 1, and oxygen is 16.
- Number of grams of atoms of carbon = 40.92/12 = 3.41
- Number of grams of hydrogen atoms = 4.58 /1 = 4.58
- Number of grams of oxygen atoms = 54.50/16 = 3.41
Step 3. Find out the atomic ratio
Find the atomic gram that is the smallest of the gram atoms we just counted. In our example, for example, it is 3.41 (carbon or oxygen -- both have the same value). You must then divide all the atomic grams by this number. Write the ratio like this: carbon value: hydrogen value: oxygen value.
- Carbon: 3.41/3,41 = 1
- Hydrogen: 4.58/3.41 = 1.34
- Oxygen: 3, 41/3, 41 = 1
- The atomic ratio is 1: 1, 34: 1.
Step 4. Convert the atomic ratio to an integer
If your atomic ratio is an integer, you can skip this step. But in the example we are using, we have to convert 1, 34 to an integer. The smallest integer that can be multiplied by a number in the atomic ratio to get an integer is 3.
- 1 x 3 = 3 (this method works because 3 is an integer).
- 1, 34 x 3 = 4 (4 is also an integer).
- 1 x 3 = 3 (again, 3 is an integer).
- The ratio in whole numbers in our example is thus carbon (C): hydrogen(H): oxygen (O) = 3: 4: 3
Step 5. Write down the empirical formula
To do this, you just need to write the letter of each element in the compound, in this case C for carbon, H for hydrogen, and O for oxygen, with the ratio values in whole numbers in the legs. The empirical formula in our example is:
C3H4O3
Tips
- The molecular formula describes the number of elements present in a compound, while the empirical formula describes the smallest ratio between the constituent atoms.
- If you were looking for the percent composition in the laboratory, you would use a spectrometer test on a sample of the compound.
