Empirical Formula Flashcards
What is a CHEMICAL FORMULA?
It is a written representation of the proportions of atoms that comprise a particular compound.
What are ways to express a chemical formula? (3)
EMPIRICAL
MOLECULAR
STRUCTURAL
What is an empirical formula?
aka SIMPLEST FORMULA.
The formula that shows the LOWEST WHOLE-NUMBER ratio of elements that comprise a compound.
The empirical formula DOES NOT equate to the actual number of atoms found in the molecule.
TRUE
The empirical formula DOES NOT equate to the actual number of atoms found in the molecule.
TRUE
What are the TWO ways to determine the empirical formula of a compound?
- Getting the ratio of the amounts of elements (in mol) present in the compound
AND
- Using the percent composition
What is percentage composition?
Mass percents of the elements from the formula of the compound.
How to compute for the PERCENTAGE COMPOSITION ALONE?
Given the compound, the subscripts are the MOLES.
- Get the moles from the subscripts of the compound.
- Individually, multiply an amount of mol by its CORRESPONDING ATOMIC WEIGHT to get the MASS of that element. (from the periodic table).
the MASS is written in GRAMS (g)
- To get the MASS of the WHOLE COMPOUND, add ALL the mass of the elements. (the one that we multiplied individually first).
- To get the MASS/WEIGHT PERCENT, the mass of an ELEMENT is divided by the mass of the WHOLE COMPOUND and then multiplied by 100.mass of an element ————————————— x 100 = mass % of that element
mass of the whole compound - To check if correct, add all the percent. It should be 100% or 99.something %
How to compute for the PERCENTAGE COMPOSITION ALONE?
Given the compound, the subscripts are the MOLES.
- Get the moles from the subscripts of the compound.
- Individually, multiply an amount of mol by its CORRESPONDING ATOMIC WEIGHT to get the MASS of that element. (from the periodic table).
the MASS is written in GRAMS (g)
- To get the MASS of the WHOLE COMPOUND, add ALL the mass of the elements. (the one that we multiplied individually first).
- To get the MASS/WEIGHT PERCENT, the mass of an ELEMENT is divided by the mass of the WHOLE COMPOUND and then multiplied by 100.mass of an element ————————————— x 100 = mass % of that element
mass of the whole compound - To check if correct, add all the percentages. It should be 100% or 99. something %
What are the FOUR steps in getting the empirical formula of a compound?
- Mass in g
- Amount in mol
- Subscript
- Divide
mass (g) -> amount (mol) -> subscript -> divide
In a word problem, the elements are NOT PRESENTED as MASS in grams instead, they are presented as MOLES. What to do?
SKIP STEP 1: MASS.
PROCEED TO STEP 2: AMOUNT.
In a word problem, the elements are PRESENTED as MASS in grams. What to do?
Get the MOLES (mol) of a certain element in the given MASS by dividing the mass to its corresponding ATOMIC WEIGHT in the periodic table.
given mass of an element (in grams) x (1 mol of the element
—————————
atomic w of the element (in grams)
= MOLES OF THE ELEMENT.
CANCEL THE GRAMS TO RETAIN THE MOL UNIT.
Then proceed in step 3 and 4.
Explain STEP 3 and 4 in getting the empirical formula.
STEP 3: SUBSCRIPTS
-Create a compound using the given elements and the moles computed during step 2 only OR step 1 and step 2.
-The moles (mol) are the subscripts of the element INDIVIDUALLY.
STEP 4: DIVIDE
-Divide the subscripts by the LOWEST subscript.
-If the quotient is in decimal, look for a number that can be multiplied to it so it will be a whole number.
-If the quotient is DECIMAL but VERY HIGH IN PLACES (.9), round off to the NEAREST WHOLE NUMBER.
In a word problem, the elements are presented as PERCENT MASS. What to do?
ASSUME 100G OF THE COMPOUND.
per 100 (PERcent)
The given percentage % is written for now as a unit of GRAMS (g).
MULTIPLY IT TO 100% (DO NOT FORGET THE PERCENT SYMBOL IN THE CALCULATOR!!!!)
That answer will be the MASS in GRAMS
then get the mole from that mass by dividing it to the atomic weight
