Stoichiometry (+ empirical formula, % yield, % comp.) Flashcards
What do you use CONSTANTLY?`
MOLES!!
What is theoretical yield?
the predicted mass of product calculated theoretically through stoichiometric calculations
What is stoichiometry based on?
the law of conservation of mass
What does “in excess” mean?
some left over
What is a limiting reactant?
a reactant that is completely used in the chemical reaction
What do you use as the base mole value for stoichiometry?
the moles of the limiting reactant
How do you identify and find the moles of the limiting reactant?
1) calculate the number of moles of both/all reactants
2) divide the mole values by the coefficient of each reactant
3) Those are the real mole values. Circle the smallest one, cross out the largest one
4) proceed wt the question
What is the assumption when doing stoichiometry?
assume the chemical reaction “goes to completion”
every single molecule of limiting reagent reacts and turns into products
What is actual/ experimental yield?
mass of product from an experiment obtained and weighted on a balance
proves that chemical reactions do not go to completion so there is some leftover limiting reagent and some product is probably lost
What is percentage yield?
measures the effectiveness of reactions by comparing the theoretical yield to the experimental yield
the more reaction steps, the more product lost and therefore decreased percentage yield
What is the percentage yield formula?
% yield = (experimental yield / theoretical yield) x 100
can use values for moles or mass but BE CONSISTENT
How is it possible to obtain percentage yields of over 100%?
when the products are contaminated, especially water, so product does not fully dry
What is percentage composition?
used to describe the percentage BY MASS of EACH ELEMENT in a COMPOUND
How do you calculate percentage composition?
(molar mass of element / molar mass of compound) x 100
What is molecular formula?
actual number of atoms of each element
What is empirical formula?
simplest whole-number ratio of elements in compound
some compounds may have same empirical formula but different molecular formulas = DIFFERENT CHEMICAL PROPERTIES
How do you determine an empirical formula?
1) identify all elements in sample
2) determine mass / element
3) use mass to determine number of moles of atoms of each element
4) determine whole number ration by dividing ALL OF THE MOLE VALUES BY THE SMALLEST MOLE VALUE
5) if necessary use multiplication to achieve whole-numbers
What do you do if there is a question about empirical formulas and you are given the percentage composition of each element but no mass value?
Assume the mass is 100g
How do you determine the molecular formula from the empirical formula?
1) divide molar mass of compound by the empirical formula molar mass to determine the multiplication factor
2) multiply each element in empirical formula by the multiplication factor
How do you determine the empirical formula from experiments?
1) deduce the mass of each element within the product
2) proceed wt calculating empirical formula (find moles, divide by smallest moles, if need to multiple for whole-number then multiply)
