Calculation of Formula or Molecular Mass
Formula mass of a compound is the sum of the atomic masses of the
all the atoms which make up the compound. However, when the compound does not have ionic bonding ,
the sum of the atomic mass can be called the molecular mass.
Are you familiar with determining the formula mass of a compound?
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By definition, 1 mole of 12C is
exactly 12 grams. One mole of 12C
has 6.022 x 1023 (Avogadro's number's)
atoms of carbon (rounded to 4 significant figures).
Are you familiar with using the "mole" as the counting unit?
Information in the Chemical Formula
Number Ratio of the Atoms
A chemical formula gives the number ratio of the different kinds of atoms present in the compound.
This means that the ratios are the same if you count in individual atoms,
dozens of atoms, or moles of atoms.
Take a look at the ratio of the number of atoms in sodium carbonate, Na2CO3.
Number Ratio of the Ions
A chemical formula gives the number ratio of the ions present in the ionic compound.
This means that the ratios are the same if you count in individual ions,
dozens of ions, or moles of ions.
Take a look at the ratio of the ions in sodium carbonate, Na2CO3.
Units of molar mass:
1 mole of carbon consists of 6.022x 1023 C atoms and weighs 12.011 g.
Are you familiar with determining the molar mass of a compound?
Take a look at the determination of the molar mass of sodium carbonate, Na2CO3.
Interconversion between Moles, grams, and quantity of particles.
Whenever you want to convert quantities from one unit to another, you need
to use the appropriate conversion factor. The conversion
factor must have units of the two quantities involved.
Molar Mass Related Calculations
Molar Mass
Units of molar mass is grams/mole
1 mole of oxygen consists of 6.022x 1023 O atoms and weighs 15.9994 g.
Conversion factor is:
* particles can be protons, electrons, atoms, ions, molecules, etc ... Conversion factor is:
* particles can be protons, electrons, atoms, ions, molecules, etc ... Conversion factors are:
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The volume of a gas is sensitive to both pressure and temperature.
If the volumes of any two gas samples are to be compared, they must be at the
same set of conditions (ie - same temperature and pressure).
The conditions that are used for reference are called standard temperature and
pressure (STP).
Molar Volume of a Gas and Related Calculations
| Do you know what they are? |
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Densities of gases are usually reported in units of grams per liter at STP.
| Click below to see the densities of a few common gases |
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For any gas at STP, when the molar mass of the gas is divided by the density of the gas, the value "22.4" is obtained.
| Take a look at the "special" number, "22.4". |
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The Molar volume of any gas at STP is 22.4 L.
| moles of the gas at STP <==> volume of the gas at STP |
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| Do you know how to determine the percent composition for a compound? | |
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| Take a look at a sample calculation. | |
To minimize errors due to rounding off numbers, keep as many significant figures in the calculations as possible.
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Percent composition establishes the relative proportions of the elements in a compound
in terms of mass. A chemical formula requires that these proportions be expressed
in terms of the number of atoms.
Example:
An analysis of a substance shows that it is made up of carbon, hydrogen, and oxygen.
The percentages of the elements are determined to be:
Determination of Empirical Formula
Determine the empirical formula of this substance.
| Answer: The empirical formula is C3H8O3. |
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We have already seen that the empirical formula of a compound
can be determined from mass data.
If the molar mass of a compound and its empirical formula
are known, the molecular formula can be determined.
The molecular formula is determined by multiplying each subscript in the empirical formula
by the number of empirical formula units.
Example
The molar mass of a compound containing only carbon and hydrogen has a molar mass of 114.0 g/mole.
Further analysis determined the empirical formula of the compound to be C4H9.
What is the molecular formula for this compound?
Determination of Molecular Formula
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Rosamaría Fong, Instructor of Chemistry,
British Columbia Institute of Technology, Burnaby, B.C. Canada.
Send Comments to Rosamaría Fong.
All contents copyrighted © 1996 Rosamaría Fong, Instructor of Chemistry All Rights reserved
: Last updated on July 19, 1996