I have published a compilation of study notes on every topic of the CIE IGCSE Chemistry course (except topic 3 which I can't seem to find). Using these study notes, I got an A* in Chemistry (well actually a 9 out of 9) and I found that without them, this would have been impossible. Each document is...
→ The empirical formula tells us the simplest mole ratio of a compound.
For example: CO2, CuSO4 are empirical, but C2H4 is not empirical
EXAMPLE
A sample of antifreeze has the following composition by mass: 38.7% carbon, 9.7% hydrogen, 51.6% oxygen. What will the empirical formula
of this compound be?
Step 1
We assume we have 100 grams of the substance; we change the percentage to grams.
Step 2
Calculate the moles of each element in the 100 grams. Moles of C: 38.7/12 =3.225
Moles of H: 9.7 /1=9.7
Moles of O: 51.6 /16 = 3.225
Step 3
Take the SMALLEST MOLE and divide every mole by the smallest. C = 3.225/3.225=1
O=3.225/3.225=1 CH3O
H=9.7/3.225 = 3.008=3
• If you get a 0.5 or half double everything (same with thirds or quarters)
Calculation 2: Molecular formula
→ The molecular formula of a compound can be larger than the empirical formula
Step 1
Calculate the rams of the empirical formula
Step 2
The molecular mass given in the equation is divided by the empirical mass to give a whole number
Step 3
multiply the small numbers in the empirical formula by the answer to Step 2
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