Textbook Ch.3: Mass Relations in Chemistry; Stoichiometry
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Atomic mass
the way that the relative masses of atoms of different elements are expressed (often referred to as atomic weights) • indicates how heavy, on the average, one atom of that element is compared with an atom of another element
What isotope is the modern atomic scale base on?
carbon-12
Atomic mass unit (amu)
a unit of mass used to express atomic and molecular weights, equal to one-twelfth of the mass of an atom of carbon-12
Isotopic abundances
atom percents, the relative proportions of the stable isotopes of each element.
Avogadro’s number
represents the number of atoms of an element in a sample whose mass in grams is numerically equal to the atomic mass of the element 6.022 * 10²³
Mole
the quantity represented by Avogadro’s number 6.022 * 10²³ of something
Molar mass (MM)
in grams per mol (g/mol), is numerically equal to the sum of the masses (in amu) of the atoms in the formula
Percent composition
specified by citing the mass percents of the elements present
What do the subscripts in a formula represent?
• The atom ratio in which the different elements are combined • The mole ratio
Simplest / Empirical formula
gives the simplest whole-number ratio of the atoms present
What do the coefficients of a balanced equation represent?
the numbers of moles of reactants and products
Excess reactant
a reactant present in an amount in excess of that required to combine with all of the limiting reactant
Limiting reactant
limits the amount of product formed
Theoretical yield
the amount of product produced if the limiting reactant is completely consumed
After you perform calculations with the two reactants, how do you know which one is the limiting reactant? What does that value represent?
the smaller of the two amounts calculated represents the theoretical yield of the product
How do you find out how much of the reactant in excess is used up in the reaction?
subtract the theoretical yield from the starting amount
Experimental yield
the amount of product that was actually collected when a chemical reaction was performed, the limiting reactant is never 100% converted into a product as the theoretical yield would assume
Percent yield
(experimental yield / theoretical yield) * 100 ** Expected to be less than 100% **