Ch 7 Flashcards
Atomic mass unit (amu)
= the unit of mass for atoms
Molecular mass
is the sum of the masses of all atoms in a molecule
Formula mass
is the sum of the masses of all atoms in a formula unit of an ionic compound (or molecule – used interchangeably)
The formula mass is
- The mass in amu of a compound
- The sum of the atomic masses of the elements in a formula
Experimentally know conversion
1 amu = 1.66053 x 10^-24 g
Avogadro’s Number
6.022 x 10^23
Avogadro’s number
- is equal to 1 mole
- Makes working with large numbers easier
Molar mass
- is the mass of one mole of molecules, atoms, ions, or formula units
- Numerically equal to the substance’s formula mass
Difference between molar mass and formula mass is units:
- Molecular weight: weight of a molecule, in amu
- Molar mass: mass of one mole of a compound, in grams
Molar mass conversion factors
- Are written from molar mass
- Relate grams and moles of an element or compound.
Percent composition
Is the percent by mass of each element in a formula.
Percent composition formula
Mass element in compound / Mass of compound x 100%
The molecular formula
Is the true or actual number of the atoms in a molecule
The empirical formula
Is the simplest whole number ratio of the atoms (this is the formula for ionic compounds)
Empirical Formula Problems
1) . Convert percentages given to grams
2) . Convert grams to moles
3) . Divide by the smallest number in order to ascertain the whole number ratio of atoms of different elements in the compound
4) . Clear any obvious fractions in step 3.
Percentages given
- Assume 100 g sample (percentages easily translate into gram quantities)
- Convert grams into moles
- Divide by the smallest number of moles present.
- If you do not have whole numbers at this point, clear the fraction by multiplying every number by the whole number that accomplishes this end.
Relating Molecular and Empirical Formulas :A molecular formula
- Is a multiple (or equal) of its empirical formula
- Has a molar mass that is the empirical mass multiplied by a whole number
molar mass = whole number x empirical mass
-Is obtained by multiplying the empirical formula by a whole number
