Molecular and Formula Mass
- the molecular mass of a molecule is the sum of the atomic masses of all the atoms in a molecular compound
- the formula mass of an ionic compound is the sum of the atomic masses of all of the atoms in an ionic compound
Determining Molecular/Formula Masses
- Determine the number of atoms of evry element present in the formula
- Look up the atomic mass of each element on the periodic table
- Multiply step one by step two for each element
- Add the products of step three and round to the appropriate number of significant digits
Example
- Calculate the molecular mass of C4H6O2.
| Element | Number | Atomic Mass of Element | Total Mass of Element in Compound |
| carbon | 4 | 12.011 amu | 48.044 amu |
| hydrogen | 6 | 1.00794 amu | 6.04764 amu |
| oxygen | 2 | 15.9994 amu | 31.9988 amu |
Molecular Mass of compound
rounded to correct number of sig figs | 86.090 amu |
Practice Problems
Determine the molecular/formula mass of the following compounds
- copper (I) chloride
- ammonium sulfide
- water
- hydrochloric acid
- dinitrogen pentoxide
Answers
Avagadro's Number
- in the above examples we have calculated the mass of molecules/ionic compounds using the relative unit, amu. Chemists need to be able to measure quantites by a standard unit of measurement such as the gram
- As we determined in lab, when using relative "weights" (amu's, rice units, etc), a gram equivalent weight of each substance will contain the same number of particles
- Avagadro determined that the number of particles necessary to convert from amu's to grams was 6.022 x 10 23
- This number of particles is the mole
- One mole (mol) of any substance contains 6.022 x 1023 particles of that substance
- One mole of any substance will have the mass, in grams, equal to the substances atomic/molecular/formula mass.
- In the above example, one mole of the compound C4H6O2 is 86.090 grams
- This quantity can be expressed using the unit grams/mol and is referred to as the gram equivalent weight of the substance or molar mass
Summary
- We can calculate the molecular/formula mass (in amu) of a substance by adding the atomic masses of all of the elements in a compound
- One mole of any substance contains 6.022 x 1023 particles of that substance
- The gram equivalent weight or molar mass of a substance can be obtained by determining its atomic/molecular/formula mass and adding the unit grams/mol
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