Chapter 3: Stoichiometry

Question 1

What is the weight of one atom of carbon 12?

Answer 1

1.99x10^-23 grams or 12 amu

Question 2

Define: atomic mass unit

- how much is 1 amu in grams?

Answer 2

a unit of mass

1 amu = 1.66x10^-24 grams

Question 3

Define: mass spectrometry

Answer 3

• experiment used to determine atomic mass
• sample is placed in a curved tube where it enters a magnetic field causing the ions to bend around the curve and hit a metal plate
• the larger the ion, the stronger the magnetic field must be to cause deflection
• this separates ions by their mass to charge ratio

Question 4

How does a luggage hand held detector check for explosives?

Answer 4

• tests for the mass of known explosives

Question 5

How do you calculate atomic weight of an element?

Answer 5

• consider the relative abundances of the isotopes and add to create the average atomic mass

Question 6

define: mole

Answer 6

unit of measurement used for counting atoms

- relates the number of atoms to a mass of a sample

Question 7

define: avogadro’s number

Answer 7

• number of atoms in 1 mole of something

- 6.022137x10^23

Question 8

Define: molar mass

Answer 8

mass in grams of 1 mole of that substance

Question 9

Define: mass percent

Answer 9

the mass of an element in a compound expressed as a % of the total mass of the compound

Question 10

Define: elemental analysis/ combustion analysis

Answer 10

• finding out the molecular composition (empircal formula) of a sample
• elemental analysis: burning a sample in the presence of O2 and determining the percent of carbon and hydrogen from the masses of CO2 and H2O

Question 11

Define: molecular formula

- how can we determine this

Answer 11

exact # of atoms in one molecule of the compound

- mass spectrometer

Question 12

Define: empirical formula

- how can we determine this

Answer 12

the formula that expresses the smallest whole number ratio of atoms in a compound
- elemental analyzer

Question 13

Define: limiting reagent

Answer 13

• reactant that is used all up

Question 14

Define: theoretical yield

Answer 14

• amount of product created from a reaction assuming all of the limiting reagent is used up

Question 15

Give the formula for percent yield

Answer 15

actual yield (g)/ theoretical yield (g) x 100%

Question 16

What are some reasons why we don’t obtain exactly theoretical yield

Answer 16

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

Question 17

Why is percent yield important to know

Answer 17

• gives information on how well the reaction is working

- change conditions of the reaction and monitor an increase or decrease in the % yield