Chem – Empirical Formula from Mass Part 3

How do you get the empirical formula from the mass of each element?

Other types of empirical and molecular formula problems require you to take the mass of each element and form them into an empirical formula.  Remember the mass in grams can always be converted to the moles of a chemical or element if we know which chemical or element we are talking about.  This conversion between mass and moles was taught in the previous section on how to convert between grams and moles.

Once we have the moles that means we have the amount of something and we can use that amount of each element to compare their ratio by dividing them (using mathematical division).


VIDEO Empirical and Molecular Formula Demonstrated Example 2: What is the empirical formula if a compound has 61 grams of magnesium 7 grams of nitrogen and 23 grams of phosphorus?  Use this periodic table if needed.


How many moles of magnesium do we have?

61g 1 mol = 2.51 mol
24.3 g

Answer: 2.51 mol Mg


How many moles of nitrogen do we have?

7g 1 mol = 0.5 mol
14 g

Answer: 0.5 mol N


How many moles of phosphorus do we have?

23g 1 mol = 0.74 mol
31 g

Answer: 0.74 mol P


What is the ratio of magnesium to nitrogen?

Magnesium = 2.51 mol = 5
Nitrogen 0.5 mol 1

Answer: 5


What does that mean?

Answer: for every 5 magnesium there is 1 nitrogen.


What is the ratio of phosphorus to nitrogen?

Phosphorus = 0.75 mol = 1.5
Nitrogen 0.5 mol 1

Answer: 1.5


What does that mean?

Answer: for every 1.5 phosphorus there is 1 nitrogen


If phosphorus = 1.5 and nitrogen = 1 and magnesium = 5 how do we create the empirical formula?

Answer: Since the empirical formula has to be all whole numbers (another word for that is integers), we have to multiply all of them by a number until we get whole numbers. Focus on those that have decimals.


What is the lowest number I can multiply 1.5 by to get a whole number?

Answer: 2


If I multiply all the numbers by 2 what do I get?


2 * 1.5 = 3 P

2 * 1 = 2 N

2 * 5 = 10 Mg


What is my empirical formula?

COMPLETE ANSWER: Mg10N2P3 (It does not matter what order you wright the elements in.)


PRACTICE PROBLEMS:  Solve the empirical formula problems below. Use this periodic table if needed.


What is the empirical formula of a compound that is 97.5g potassium and 46.2g silicon?

Answer: K3Si2


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