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Regents Chemistry Topic Review: Stoichiometry/ Chemistry Calculations

Calculating Average Atomic Mass

An average atomic mass is calculated by taking the mass of each isotope, multiplying it by the % abundance and adding all the isotopes together.

Example: Calculate the average atomic mass of silicon, given the following data:



Average atomic mass = 27.97693amu*.9223 + 28.97649amu*0.0467 + 29.97376amu*0.0310

Average atomic mass = 28.1 amu








Calculating Formula Mass

Formula Mass is calculated by multiplying the number of atoms of each element (subscript) by the mass from the Periodic table and adding all of the elements up.

For example, formula mass of H2O is

H = 2*1g/mol = 2g/mol

O = 1*16g/mol = 16g/mol

H2O = 2+16=18 g/mol


Mole Concept

Table T on the Reference Table provides a formula for calculating moles.

Number of moles = given formula/ gram-formula mass


What is the number of moles of CO2 in a 220.-gram sample of CO2 (gram-formula mass = 44 g/mol)?

Solution: Table T on the Reference Table shows the formula for mole calculations. number of moles = given mass/ gram-formula mass.

Number of moles. = 220g/44g/mol = 5.0moles


% Composition by Mass

The formula for % Composition by Mass can be found in Table T on the Reference table.

% composition by mass = (mass of part/ total mass)*100


Empirical Formula Versus Molecular Formula

An empirical formula is a formula with the simplest ratio of atoms (subscripts can not be divded any further). A molecular formula is an actual formula for the molecule and has multiples of empirical formula going into it. For example, C2H4 is a molecular formula and can be simplified further (both subscripts can be divided by 2) into CH2, which is am empirical formula.


Conservation of Mass and Conservation of Charge

According to conservation of mass, the total mass of reactants must be equal to the total mass of products.

According to the conservation of charge, the total charge on the reactant side must be equal to the total charge on the product side.


Mole to Mole Problems Using A Balanced Equation


The coefficients in a balanced equation, represent mole to mole ratios.


For example: Given the equation representing a reaction: 2C + 3H2 → C2H6 What is the number of moles of C that must completely react to produce 2.0 moles of C2H6 ?

Solution: The coefficients in the equation are 2 for C and 1 for C2H6 (if the coefficient is not shown, we can assume that it is 1). This means when 2 moles of C react they produce 1 mole of C2H6. Therefore, to produce 2.0 moles of C2H6, we would need twice the amount of C, which would be 2*2 = 4moles


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