Chapter 7

Formula mass - the mass of a single molecule or formula

  • “Formula weight,” “Molecular mass,” or “molecular weight”

Percent Composition

==Mass of one element/mass of compound x 100%==

How chemists measure formula mass and percent composition

==Mass spectrometry== - a technique used to measure the mass of molecules

  1. Ionizing chamber
  2. Electric field
  3. Detector
  • Used to monitor for hazardous or illegal substances

==Elemental analysis== - a technique used to measure the percent position, uses combustion reaction to form simpler products

The mole concept

==One atomic mass unit (u) = 1.66 x 10^-24 g==

1 mole: 6.02 x 10^23 of anything

  • Avagandro’s number

MolerelatesatomstogramsMole relates atoms to grams

Grams per mole = molar mass

Converting between grams and moles

Use molar mass as the conversion factor

Converting between moles and particles

Use ==Avagadro’s number: 6.02 x 10^23 particles = 1 mole==

Relating atoms to grams

  1. Convert from grams to moles using molar mass
  2. Convert moles to atoms using Avogadro’s number

The mole concept in balanced equations

==Stoichiometry== - Using the amount of one material to predict the amount of another, based on the balanced equation

Gram-to-Gram Problems

  1. Convert the grams of substance A to mole of substance A using molar mass
  2. Relate the moles of substance A to the moles of substance B using the mole ratio from the equation
  3. Convert from the moles of substance B to grams of B using the molar mass

Strategies for Solving Stoichiometry Problems

Grams and moles of one substance - molar mass

Moles and particles of one substance - Avogadros’s number

Moles of two different substances - mole ratio from the balanced equation

The Mole Concept in Balanced Equations: Limiting Reagents

Calculation with Limit Reagents

Limiting Reagent - runs out first, limits the amount that can be produced

Excess reagent - Not completely consumed; reagent will be left over after the reaction is complete

I - Initial (How much do we have at the beginning?)

C - Change (How much is consumed or produced during the reaction?)

E - End (The end amount)

Theoretical and Percent Yield

Theoretical Yield - the amount of product that can form, based on the balanced equation

Actual Yield - The amount actually obtained

Percent Tield - The percentage of the theoretical yield that was obtained

  • Actual yield/Theoretical yield x 100%
Why is the actual yield so low?
  • The material sticks to the container walls
  • Unwanted side product
  • Product is lost during the purification

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