Molecular Formula Empirical Formula and Calculation

Molecular formula, empirical formula and calculation are classical exam questions. It never die out as favorites of the examiner's attention. What is molecular formula, empirical formula ? And calculation for? They are actually a method for many chemist to figure what they are studying in their test tubes. Most of time, or accidentally some compounds were form, and this is the one way to know what the possibility is this compound may look up in the formula. The real form is "molecular formula" the simplified form is "empirical formula"

Let's start by working in this questions... knowing that Moles= Mass/ Molecule Weight

Questions: what is the empirical formula, molecular formula of the following compound has C= 40%,  H=6.67%,  and MW = 90 g/mole? (give molecular formula, empirical formula and calculation).

Step1 : First we have to think what is in this compound, C, H and ?? anything else? by calculating the % ...

40% + 6.67% only = 46.67 %

Where is the rest of 53.3%?

That must be Oxygen, so now we can know the compounds were composed of carbon, hydrogn, and oxygen. ( why is oxygen, because you are studying organic chemistry that you know if we have to have C, H and O in "Organic Chemistry")

Let's line it up

C= 40%

H= 6.67%

O=53.3%

Since the empirical formal is the ratio between C, H, and O

Ratio can be obtain if we have moles

We can come up moles if we know the Mass

(Ratio-> Moles-> Mass) so now we are working backwards ... ( Mass -> Mole -> Ratio)

We can assume then that we have 100g of compound so we can have moles and ratio. ( yes, you can do that... because in the end we will get ratio, not really a value that is measured.)

C =40%  ==> 40 g

H = 6.67%  ==> 6.67 g

O = 53.3%  ==> 53.3 g

 can we figure out the moles?

Yes we can by divide the gram by Molecular weight.

C  40 g / 12g/mole = 3. 33 mole

H  6.67 g / 1g/mole = 6.67 mole

O  53.3 g/ 16g/mole = 3.33 mole

Now we can write

But no one in the science world will read it

So...

We divide the 3.33 by 3.33= 1 and everything else divide this smallest number to get the proportional number

1:2:1  ==> this is the smallest ratio = empirical formula

How about the molecular formula given in the questions? Molecular Weight = 90 g/mole

Is  MW of  = 90? no  it is only =30 g/mole

Think for a moment .... what is the right answer?

 

... .....

 

() 3

Now after you have done so great on this questions... let's see again what is this method.

Mass Percentage

Mass percentage is a simple method how to convert the mass ratio and for us to assume - let's see the example below later but bear in mind that this is always the first thing that we use on the exam. 

Empirical Formula

Empirical Formula is what the test results gives you. Usually just the ratio we can obtain from the experimental equipment. It would be nice to get the real formula, but not in most case. For exam questions, we will find out empirical formula before molecular formula but that does not mean this empirical formula is not molecular formula. They can be exactly the same!

Molecular Formula

Molecular formula is what you see is what you get. One classical question will be address in the following examples. The question would not give you this.. this is the answer of the questions.

Appendix:

  The ratio of 3:6:2 means this compound was made of 3 carbon, 6 hydrogen and 2 oxygen

In other words, 1 moles of the compound was made of 3 moles of carbon, 6 moles of hydrogen and 2 moles of oxygen

Molecule Weight unit = g/mole. that means in one mole, you have certain gram. 1 mole =6.02*10 ^ 23 individual molecules.