Atoms, Molecules and Stoichiometry


1. Relative masses of atoms and molecules
2. The mole ,Avogadro Constant
3. Calculation of Empirical and Molecular Formulae
4. Reacting masses and volumes (of solution and gases)



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Atomic Abundance Example Chemistry Problem:

The element boron consists of two isotopes, ¹⁰₅B and ¹¹₅B. Their masses, based on the carbon scale, are 10.01 and 11.01, respectively. The abundance of ¹⁰₅B is 20.0%.
What is the atomic abundance of and the abundance of ¹¹₅B?

Solution

The percentages of multiple isotopes must add up to 100%.
Since boron only has two isotopes, the abundance of one must be 100.0 - the abundance of the other.

abundance of ¹¹₅B = 100.0 - abundance of ¹⁰₅B

abundance of ¹¹₅B = 100.0 - 20.0
abundance of ¹¹₅B = 80.0

Answer

The atomic abundance of ¹¹₅B is 80%


http://chemistry.about.com/od/workedchemistryproblems/a/abundanceexampl.htm

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No. of mole= Mass divided by Mr

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To get the percentage composition from formulas, we calculate the percent of each element present,

as follows:

% Element X = atomic weight of the element X number of atoms of that element _X 100%

Formula weight of the compound

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The theoretical yield is the maximum amount of product possible from the available reactants. We can calculate

the theoretical yield, and we are usually given the actual yield. The definitions and procedure for calculating the % yield are shown below.

1. Theoretical Yield - the amount of product predicted by the balanced equation based on the amount of the limiting reactant.

2. Actual Yield - the amount of product obtained when the reaction is actually carried out in the laboratory.

3. Per Cent Yield:

% Yield = Actual yield X 100%

Theoretical yield