Determining Empirical Formula - Weebly
Determining Empirical Formula
Empirical means lowest or simplest.
Empirical Formula is the lowest whole number ratio of atoms in a compound
Examples: C2H4 would be CH2
The way we mathematically find the empirical formula is fairly simple, it just takes a few steps.
STEP 1
Convert given units to moles of each element.
• These givens may be in the form of grams (g), percent % (assume 100g of substance) , or already in moles (
STEP 2
Divide the moles from STEP 1 by the smallest number of moles in the set. This will give you the whole number ratio (subscript) for each element in the compound.
STEP 3
Rounding the ratio number. You may only round up or down if the number is extremely close to the next whole number, i.e. 1.98 = 2, 3.01=3. Note: 1.78 is NOT = 2
• If the ratio number ends in .3 or .6 (1/3), multiply by 3
• If the ratio number ends in .24 or .75 (1/4), multiply by 4
• If the ratio number ends in .5 (1/2), multiply by 2
REMEMBER WHEN MULTIPLYING A NUMBER IN A RATIO, YOU MUST MULTIPLY ALL NUMBERS BY THE SAME MULTIPLIER.
Determining Molecular Formula
Molecular Formula is the actual number of atoms that bond together in a compound.
• In solving Molecular Formula problems the Molecular Mass (MM) or Molecular Weight (MW) must be given in the problem.
STEP 1
Find the empirical mass. This is done the same way as formula mass only it is called the empirical mass because it is derived from the empirical formula.
• Note: You may have to do an empirical mass problem in order to find the empirical formula.
STEP 2
If the multiplier number is not even as in the STEP 3 above follow the same instructions to get your closest whole number multiplier.
STEP 3
Divide Molecular Mass by Empirical Mass. (MM/EM). This number is you multiplier for the empirical formula. EF= CH2 multiplier is 2 so.. CH2 x 2 = C2H4
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