*
P. Perkerson
* Empirical formula is defined as the smallest
whole number ratio of atoms in a formula.
* Ionic compounds will usually(but not always)
be empirical but covalent compounds often
are not
* Ex.
*
MgCl2 a 1:2 ratio
C2H6
2:6 ratio – not the smallest
whole number ratio, so not empirical. Simplify
to CH3 and that is the empirical formula
* When an unknown substance is analyzed in a
crime lab, the mass spectrometer gives the
percent of each element present in the
sample. From that information the empirical
formula can be calculated.
* Keep going and let’s see how that works!
* When percents are given, we assume that we
have 100 grams of the sample. That makes it
easy to convert the percents to grams
because percent means “parts per hundred
parts”!!
* Analysis shows a substance is composed of
38.43% Mn, 16.80% C, and 44.77% O so…
* 38.43g Mn
* 16.80 g C
* 44.77 g O
* Next step is to convert grams to moles
* 38.43g Mn
* 16.80 g C
* 44.77 g O
*
1 mol Mn
54.94g Mn
1 mol C
12.01g C
1 mol O
16.00 g O
= 0.6995 mol Mn
= 1.399 mol C
= 2.798 mol O
*Now we divide all by the small! In other
words, divide each mole value by the
smallest mole value. If the number is
within one tenth of a whole number round
it. If it is NOT within a tenth, you have
to multiply each by some number so that
they are all whole numbers and those
become the subscripts in the formula.
* 0.6995 mol Mn/ 0.6995 = 1
* 1.399 mol C/ 0.6995 = 2
* 2.798 mol O/ 0.6995 = 4
* The empirical formula is MnC2O4
* Conveniently, all of those numbers could be
rounded to a whole number. But what if one
of them came out to be 1.5? Simple, just
multiply everything by 2. What it if was 1.25
or 1.33?
* Your turn!
* Unknown substance is analyzed and found to
be composed of 34.53% zinc, 14.79%
nitrogen, and 50.68% oxygen. Calculate the
empirical formula. Show all work!!
* For many compounds, particularly covalent,
the empirical formula is not the true formula.
The molecular formula is always a whole
number multiple of the empirical formula. To
determine what that multiplier would be,
divide the molecular formula mass by the
empirical formula mass.
* Ex. Maleic acid has a molecular formula mass
of 116.10 g/mol and an empirical formula mass
of29.02 g/mol and the empirical formula is
CHO
* Divide 116.02 by 29.02 =4 (3.9979) so 4 is
the multiplier and the molecular formula for
maleic acid is C4H4O4
* Your turn! Silver oxalate has a molecular
molar mass of 303.80 g/mol and is composed
of
* 71.02% silver
* 7.91% carbon
* 21.07% oxygen
* Determine the empirical and molecular
formulas.