Chapter 3 - Stoichiometry

Question 1

Try the example in chapter 3, slide 7 about mass spectrum of chlorine

Answer 1

or try the loncapa question! know how to do it

Question 2

try the mass spectrum of molecular parent ion question on notes chapter 3 on slide 10

Answer 2

answer below notes

Question 3

What is the avogadro’s number

Answer 3

6.022137*10^23 atoms

• one mole of anything consists of 6.022137*10^23 units of that substance

Question 4

What is a molecular formula?

Answer 4

• exact # of atoms in one molecule of the compound

Question 5

What is the empirical formula?

Answer 5

• the formula that expresses the smallest whole number ratio of atoms in a compound

Question 6

The compound adrenaline contains 56.79% C, 6.56% H, 28.37% O, and 8.28% N by mass. What is the empirical formula for adrenaline? How to interpret these data?

Answer 6

Use 100g sample

56.79g/12.011g/mol = 4.728 mol C / 0.591 mol = 8

6.50g/1.008g/mol = 6.51 mol H / 0.591 mol = 11

28.37g/15.999g/mol = 1.773 mol O / 0.591 mol = 3

8.28g/14.01g/mol = 0.591 mol N / 0.591 mol = 1

after getting the moles, divide each by the smallest # mole to get ratio

therefore, C8H11O3N is the empirical formula
- molecular formula can be a multiple of this

Question 7

What are the steps for calculating the masses of reactants and products? (REVIEW)

Answer 7

• balance the equation for the reaction
• convert the known mass of the reactant or product to moles
• using the equation, use the required mole ratio to calculate the number of moles of the desired reactant or product
• convert moles to grams

Question 8

What is another word for a “predicted” amount of product?

Answer 8

Theoretical yield

Question 9

What is the theoretical yield dictated by and why

Answer 9

• dictated by the limiting reagent
• theoretical yield assumes 100% completion, in other words, all of the limiting reagent is used up

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Question 10

What is the actual yield of a product?

Answer 10

• obtain less product than the theoretical yield
• usually given as a % of the theoretical yield
• this is called percent yield

Question 11

Why have a limiting reagent?

Answer 11

• expensive
• difficult to prepare in sufficient quantity
• could be both
• using an excess of cheaper or more readily available reactant “ensures” complete use of the limiting reagent
• separation of the product from the reactants is a consideration

—- if one of the reactants is less easily separated from the product(s) then make that reactant limiting

Question 12

REVIEW - steps for balancing equations

Answer 12

1. determine what reaction it is. List reactants and products (with physical states_
2. write the unbalanced equation to summarize the reaction
3. balance the equation by inspection, starting with the most complicated molecules
4. determine what coefficients are necessary to balance the # of each type of atom on either side of the equation
5. do not change any chemical formula
6. coefficients should be whole numbers
7. must have the same number and type of atoms on each side

Question 13

Give 4 reasons why the maximum amount in theory may be limited.

Answer 13

• reaction conditions may not be optimum
• reactions may be very slow
• side reactions may occur
• difficult to isolate all products in pure form

Question 14

What does percent yield give us

Answer 14

• gives information on how well the reaction is working
• change conditions of the reaction and monitor an increase or decrease in the % yield

Question 15

REVIEW - steps for solving stoichiometric problems

Answer 15

1. balance equation
2. convert known masses to moles
3. determine limiting reagent
4. using the limiting reagent, determine the # of moles of the desired product
5. convert moles to grams