empirical and molecular formulas how to find out what an unknown compound is
TRANSCRIPT
A chemist obtains a new product
What is the formula for the compound?
First step – determine constituent elements and their amounts
This info can be used to determine chemical formula
Formula of a compound represents relative numbers of atoms present
E.g. “CO2” tells us that in a molecule of this compound there is 1 carbon atom to every 2 oxygen atoms
To determine the formula of a substance we need to count the atoms – we can do this by weighing
An example
You have a compound that you know contains only Carbon, Hydrogen and Oxygen.
You have a 0.2015 g sample
Analysis shows you have 0.0806 g C, 0.01353 g H, and 0.1074 g O
We can convert these masses to moles, and then moles to atoms using dimensional analysis…..
To summarize up to this point
We have .00671 moles of Carbon
We have .01342 moles of Hydrogen
We have .006713 moles of Oxygen
Amount of Carbon = Amount of Oxygen
.01342/.006713 = 2, so we have twice as much Hydrogen as we have of Oxygen and as we have of Carbon
We have a ratio of 1:2:1
We know the ratio of elements
1 Carbon : 2 Hydrogens : 1 Oxygen
We can write this as CH2O
Is this the molecular formula? Maybe…but the molecule might also have 2 Carbons, 4 Hydrogens and 2 Oxygens, or it might have 16 Carbons, 32 Hydrogens and 16 Oxygens
We have found the EMPIRICAL FORMULA – a formula that represents the ratio of elements in a compound.
This is also called “simplest formula” since it is smallest whole-number ratio of elements in the compound
Empirical Formula Vs Molecular Formula
Empirical formula gives relative numbers of atoms e.g. CH2O
Molecular formula gives the actual numbers of atoms e.g. C6H12O6
C6H12O6 = (CH2O)6
How to calculate empirical formula
We have 0.2636 grams of nickel. We heat it in the presence of oxygen to produce 0.3354 grams of a nickel oxide. What is the formula of the compound we made?
First - what is the mass of oxygen that reacted with the copper?
Mass of copper oxide – Mass of copper = mass of oxygen
0.3354 g copper oxide – 0.2636 g copper = 0.0718 g oxygen
Next – Find number atoms involved…
Find number of atoms in the compound
•Mole quantities represent number of atoms•We have same number of moles of oxygen as of nickel•Empirical formula will be NiO
Another example
You have a metal oxide made by reacting 4.151g Al with 3.692 g O. What is the empirical formula?
Atomic mass Al = 26.98 g/mol
Atomic mass O = 16.00 g/mol
Need to know relative numbers of atoms, so need to convert grams to moles and then find the whole number ratio of atoms
Find whole number ratios
Do this by dividing both numbers by the smallest of the two. This converts the smallest number to 1
This is not a whole number ratio – to get to a whole number, all we have to do is multiply by 2
If you have percent composition your can also find the empirical formula
You have a compound that is 27% Carbon and 73% Oxygen by mass. What is the empirical formula?
Assume you have 100 grams. That means you would have 27 g C and 73 g O
Convert grams to moles:
27 g C (1 mol/12 g C) = 2.25 mol C
73 g O (1 mol/16 g O) = 4.6 mol O
Divide by smallest # of moles:
4.6 /2.25 = 2.04 (O)
2.25/2.25 = 1 (C)
Empirical formula = CO2
Find empirical formula from percent composition
Nylon-6 is a compound that is 63.68 C, 12.38% N, 9.80% H and 14.4% O. Find the empirical formula
C6H11NO
One more step to find molecular formula
To find the molecular formula of your unknown compound, you need another piece of information – the molar mass of the compound, in addition to the percent composition
Finding molecular formula
The molar mass of the compound will be some multiple of the empirical mass – find the multiple by dividing molar mass by empirical mass:
(238.88 g/mol)/(141.94 g/mol) = 1.7
1.7 ≈ 2
Molar mass of unknown is about twice empirical mass. We will multiply empirical formula subscripts by 2
Molecular formula = P4O10