Empirical + Molecular Formula

Today's lesson is on empirical and molecular formula.

Empirical Formula = gives the lowest term ratio of atoms (or moles of course) in the formula.
Ionic compounds are empirical formulas as well!

And here we have our example:

Lets say we have 10.87 g of Fe and 4.66 g of O. What is the empirical formula?
Well, first we convert grams to moles.

we want to convert the 10.87 grams so we will put that first. We want to eliminate grams so be sure to put the molar mass of the element in grams on the bottom.
10.87 g x 1 mole
                ------
                55.8 g = 0.195 moles
4.66 g x 1 mole
              -----
              16 g = 0.291 moles

Now the smaller one of the two is the first one, so be use that one and divide by that amount.
0.195/0.195 = 1
0.291/0.195 = 1.5
Now we have to scale ratios to whole numbers. We need to get rid of that 1.5. We do that by multiplying both by two, because then the 1.5 turns into a 3!
Fe 1 x2 --> 2
O  1.5 x2 ---> 3
So the answer would be Fe2O3

Here is a problem for you to try for practice:
A compound used for sweetening is 57.14% C, 6.16% H, 9.52% N, and 27.18% O.  Calculate the empirical formula of the sweetener and find the molecular formula.  (The molar mass of the compound is 294.30 g/mol)
Remember to follow the steps!
1) convert to moles
2) divide both by the smallest molar amount
3) scale ratios

Now onto molecular formula:

Lets use this formula to help answer the questions:

n = molar mass of compound
      --------------------------
      molar mass of the empirical formula

The empirical formula of a gas is CH2. What is the molecular formula if the molar mass is 42 g/mol?

First, lets calculate the molar mass of the compound, because we need it in the equation.
The mass of carbon is 12, and the mass of hydrogen is 1, but there is 2 o them. So the molar mass of the compound is 14.

Now lets plug that into the equation!

n = 42 g/mol
      --------
      14 g/mol
which equals 3 g/mol.
Now write the answer 3(CH2)
make sure to distribute the 3.
so it is actually C3H6 = MF propane
Make sure you write the MF! It stands for molecular formula. You can remember it by thinking that it is Melanie's initals!

If you want a little extra help, I've got videos galore for you!
Molecular and Empirical Formula
A helpful chemistry tutorial
Extra help!
A super cool 16 minute tutorial! woah!