Download Atomic Masses

Atomic Masses
The most accurate method of comparing masses of atoms is by using the mass spectrometer.
The mass spectrometer knocks electrons off the atoms or molecules being analyzed and changes
them into positive ions. An applied electric field then accelerates these ions into a magnetic field.
An accelerating ion produces its own magnetic field, which tends to change the path if the ion.
The amount of path deflection for each ion depends on its mass.
The atomic mass of any element is a mixture of isotopes, thus the atomic mass we use for that
element is an average value reflecting the average of the isotopes composing it.
(%of isotope1)(Massofisotope1)+(%ofisotope2)(Massofisotope2)+…
Most elements occur in nature as mixture of isotopes, thus atomic weights (masses) are usually
average values.
The masses for each element listed in the table inside the front cover of your chemistry textbook
are an average value based on the isotopic composition of the naturally occurring element.
The mole's numeric value is 6.022x1023 One mole of something consists of 6.022x1023 units of
that substance.
The mass of 1 mole of an element is equal to its atomic mass in grams.
Molar Mass
The molar mass of a substance is the mass in grams of one mole of the compound, and is found
by adding the average atomic masses of each atom in the compound.
Percent Composition of Compounds
We can obtain the mass percent of the elements from the formula of the compound by comparing
the mass of each element present in 1 mole of the compound to the total mass of 1 mole of the
compound.
Empirical Formula Determination (simplest whole number ratio)
• Since mass percentage gives the number of grams of a particular element per 100 grams of
compound, base the calculation on 100 grams of compound. Each percent will then represent
the mass in grams of that element
• Determine the number of moles pf each element present in 100 grams of compound using the
atomic masses of the elements present
• Divide each value of the number of moles by the smallest of the values. If each resulting
number is a whole number (after appropriate rounding), these numbers represent the
subscripts of the elements in the empirical formula
• If the numbers obtained in the previous step are not whole numbers, multiply each number by
an integer so that the results are all whole numbers
Molecular Formula Determination (actual number of atoms present.)
• Obtain the empirical formula
• Compute the mass corresponding to the empirical formula
• Calculate the ratio
Molar mass
Empirical formula mass
• The integer from the previous step represents the number of empirical formula units in one
molecule. When the empirical formula subscripts are multiplied by this integer, the
molecular formula results. This procedure is summarized by the equation:
Molecular formula = (emp. formula) x (Molar Mass/Emp. Form.Mass)
Chemical Equations
A chemical change involves a reorganization of the atoms in one or more substances.
By a chemical equation with the reactants on the left side of an arrow and the products on the
right side
Bonds have been broken, and new ones have been formed. It is important to recognize that in a
chemical reaction, atoms are neither created nor destroyed. All atoms present in the reactants
must be accounted for among the products. By balancing a chemical equation, there must be
the same number of each type of atom on the product side and on the reactant side of the
arrow.
A chemical equation often gives the physical states of the reactants and products:
State
Symbol
Solid
s
Liquid
l
Gas
g
Dissolved in water (in aqueous solution)
aq
The relative numbers of reactants and products in a reaction are indicated by the coefficients in
the balanced equation
Writing and Balancing the Equation for a Chemical Reaction
• Determine what reaction is occurring. What are the reactants, the products and the physical
states involved?
• Write the unbalanced equation that summarizes the reaction described above
• Balance the equation by inspection, starting with the most complicated molecules. Determine
what coefficients are necessary so that the same number of each type of atom appear on both
reactant and product sides. Do not change the identities (formulas) of any of the reactant or
products
Calculating masses of reactants and products in chemical reactions:
--Balance an equations for the reaction.
--Convert the given quantity or quantities to moles.
--If there is more then one given reactant, determine which is limiting reactant. Divide moles by
coefficient…smaller is limiting)
--Use the balanced equation to convert moles of limiting reactant or given quantity to moles of
desired substance
--Convert to the units of desired substance.