Stoichiometry (+ empirical formula, % yield, % comp.)

Question 1

What do you use CONSTANTLY?`

Answer 1

MOLES!!

Question 2

What is theoretical yield?

Answer 2

the predicted mass of product calculated theoretically through stoichiometric calculations

Question 3

What is stoichiometry based on?

Answer 3

the law of conservation of mass

Question 4

What does “in excess” mean?

Answer 4

some left over

Question 5

What is a limiting reactant?

Answer 5

a reactant that is completely used in the chemical reaction

Question 6

What do you use as the base mole value for stoichiometry?

Answer 6

the moles of the limiting reactant

Question 7

How do you identify and find the moles of the limiting reactant?

Answer 7

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

Question 8

What is the assumption when doing stoichiometry?

Answer 8

assume the chemical reaction “goes to completion”

every single molecule of limiting reagent reacts and turns into products

Question 9

What is actual/ experimental yield?

Answer 9

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

Question 10

What is percentage yield?

Answer 10

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

Question 11

What is the percentage yield formula?

Answer 11

% yield = (experimental yield / theoretical yield) x 100

can use values for moles or mass but BE CONSISTENT

Question 12

How is it possible to obtain percentage yields of over 100%?

Answer 12

when the products are contaminated, especially water, so product does not fully dry

Question 13

What is percentage composition?

Answer 13

used to describe the percentage BY MASS of EACH ELEMENT in a COMPOUND

Question 14

How do you calculate percentage composition?

Answer 14

(molar mass of element / molar mass of compound) x 100

Question 15

What is molecular formula?

Answer 15

actual number of atoms of each element

Question 16

What is empirical formula?

Answer 16

simplest whole-number ratio of elements in compound

some compounds may have same empirical formula but different molecular formulas = DIFFERENT CHEMICAL PROPERTIES

Question 17

How do you determine an empirical formula?

Answer 17

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

Question 18

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?

Answer 18

Assume the mass is 100g

Question 19

How do you determine the molecular formula from the empirical formula?

Answer 19

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

Question 20

How do you determine the empirical formula from experiments?

Answer 20

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)