4.6- Stoichiometry in the Real World - Excess/Limiting Amounts, Percent Yield and Impurities

Question 1

What are 3 assumptions made about reactions?

Answer 1

• Reactants are 100% consumed.
• Reactants are 100% pure.
• Only 1 reaction occurs at a time.

Question 2

Why are reactants not all 100% consumed?

Answer 2

Only 1 reactant is 100% consumed, some quantities of the other reactants always remain.

Question 3

Why aren’t reactants 100% pure?

Answer 3

Depending on the chemicals grade or quality, unavoidable impurities find their way in and change the mass of the reactant slightly.

Question 4

Why doesn’t only 1 reaction happen at a time?

Answer 4

Some of the excess reactants not consumed by the main reaction can form side-reactions and create by-products.

Question 5

What do you call the mole ratio from a balanced equation, representing the stoichio relationship between 2 substances (not actual ratio mixed in experiment)

Answer 5

Stoichiometric ratio

Question 6

T or F, reactants are always combined in their exact stoichiometric ratio

Answer 6

False, it is impossible to be that precise.

Question 7

What two pieces of information are needed about a reaction to calculate?

Answer 7

Which reactant has a smaller amount and the ratio between the substances.

Question 8

What do you call the reactant that is totally consumed by the reaction

Answer 8

Limiting reactant

Question 9

What do you call the reactant of which some is left over when the limiting reactant runs out and the reaction is complete, as there was more of it present than necessary?

Answer 9

Excess reactant.

Question 10

Why can’t you just compare the masses of two reactants to figure out which is limiting?

Answer 10

Because different substances have particles of different sizes and masses, meaning a smaller mass of an element could contain more particles.

Question 11

To compare 2 substances through stoichiometric calculations, you must know what?

Answer 11

The mole ratio between them.

Question 12

How do you determine which reactant is limiting?

Answer 12

Find how many moles of each reactant you have, compare mole ratio to stoichiometric ratio of balanced chem equation.

Question 13

What two stoichiometric calculations do we regularly use in the lab?

Answer 13

Compare limiting reactant to another reactant / Compare the limiting reactant to one of the products.

Question 14

Comparing the limiting reactant to another reactant helps to find what?

Answer 14

How much excess reactant will react or how much will be left over.

Question 15

Comparing the limiting reactant to one of the products helps to find what?

Answer 15

How much product you expect to form, theoretical yield.

Question 16

What do you need to use in calculation to determine how much excess reactant will react?

Answer 16

the mole ratio from the balanced chemical equation.

Question 17

How do you determine how much excess reactant will be over after a reaction?

Answer 17

Determine how much you had and how much reacted, then find difference between them.

Question 18

T or F, all reactants are 100% pure

Answer 18

False.

Question 19

What do you call the percentage expressing the ratio by mass of how many grams of pure compound are present in a sample ?

Answer 19

Percent purity.

Question 20

For percent purity, the question will give 2/3 of what info and ask you to find the 3rd?

Answer 20

• Mass of pure compound
• Mass of impure sample
• Percent purity

Question 21

What is the rearranged formula for percent purity?

Answer 21

mass of impure sample x percent purity / 100% = mass of pure compound.

Question 22

How do you incorporate purity of sample into stoichiometric calculations?

Answer 22

Start with impure mass, then multiply by decimal less than one equal to value of purity, over 100%.

Question 23

Why might a reaction not produce the amount of product anticipated?

Answer 23

• side reactions

- lack or precision in measurements.

Question 24

What do you call the percentage expressing the ratio of how much product you actually obtain from the reaction to how much you expected?

Answer 24

Percent yield.

Question 25

To calculate the percent yield of a reaction, you need to know what?

Answer 25

The theoretical yield.

Question 26

What do you call the amount of product that you expect to form based on the stoich ratio of the reaction and the complete consumption of limiting reactant?

Answer 26

Theoretical yield.

Question 27

Percent yield is most often based on what measurement?

Answer 27

Mass (not for gas)

Question 28

When determining percent yield of gases, use what form of measurement?

Answer 28

Volume.

Question 29

How do you calculate theoretical yield based on stoich calculations?

Answer 29

Use mass of limiting reactant, balanced chem equation to calculate how much product should form.