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10.1 Mole: a Measurement of Matter

10.1 Mole: a Measurement of Matter. After completing this section, you will be able to: Describe methods of measuring the amount of something Define Avogadro’s number as it relates to a mole of a substance Distinguish between the atomic mass of an element and its molar mass

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10.1 Mole: a Measurement of Matter

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  1. 10.1 Mole: a Measurement of Matter • After completing this section, you will be able to: • Describe methods of measuring the amount of something • Define Avogadro’s number as it relates to a mole of a substance • Distinguish between the atomic mass of an element and its molar mass • Describe how the mass of a mole of a compound is calculated

  2. Chemistry is a quantitative science • Measure the amount of a substance in a chemical reaction • Measure matter by: • 1. count the number of something you have • 2. determine the mass • 3. measure the volume

  3. Able to convert from one form of counting to another

  4. Easy to count the number of large items • No way to count the number of small things like atoms or particles • Easier if the atoms are grouped • Reason why chemists use a mole • 1 mole = 6.02 x 1023 representative particles • SI unit for measuring the amount of a substance • Called Avogadro’s number • Representative particle is the item in a substance • Element’s representative particle is the atom • Molecular compounds and diatomic elements have a molecule • Ionic compound’s representative particle is formula unit

  5. Conversion factor for particles to mole • Mole = given rep part x 1mole/6.02 x 1023 rp

  6. Convert from moles to particles • Need to make sure that the number of particles in the given compound and the number of particles asked for the question are the same. • Sometimes need to know how many atoms are in a molecule or formula unit • Ex: CO2 has 3 atoms • In one molecule of CO2, there are 3 x Avogadro’s number of atoms

  7. Rep part = given mole x 6.02 x 1023 rp / 1 mole

  8. Mass and mole are related • Atomic mass of an element is expressed in atomic mass units • Grams are more convenient in the lab • Chemists have converted masses in amu to grams • Atomic mass of an element expressed in gram is the mass of a mole of the element • Mass of one mole of an element is called the molar mass • Atomic mass rounded to one decimal place • Molar mass of any element contains one mole or 6.02x1023atoms of that element

  9. Mass of a mole of a compound • To find the mass of a mole of a compound, need to know the formula of the compound • Sulfur trioxide is SO3 • Has 1 sulfur and 3 oxygen • 1 sulfur = 32.1 = 32.1 • 3 oxygen = 3(16.0) = 48.0 80.1 g/ mole

  10. 10.2 Mole-Mass and Mole-Volume Relationships • After completing this section, you will be able to: • Describe how to convert the mass of a substance to the number of moles of a substance and moles to mass • Identify the volume of a quantity of a gas at STP

  11. Mole-mass relationship • Molar mass is the mass in grams of one mole of that substance • Allows you to convert between mass of a substance and mole of a substance • Convert from mole to mass • Mass (g) = number of mole given X mass (g) / 1 mole

  12. Convert from mass to moles • Moles = mass(g) of given X 1mole/mass (g)

  13. Mole – volume relationship • The volume of moles of a gas are much more predictable than solids and liquids • Avogadro’s hypothesis explains this • Equal volume of gases at the same temperature and pressure contain equal number of particles • Properties of gases • Large amount of space between particles • Volume of gas varies with pressure and temperature • Volume of a gas is usually measured at standard temperature and pressure (STP) • Temperature of 0°C and pressure of 1atm

  14. At STP, 1 mole or 6.02 x 1023 rp, of any gas occupies a volume of 22.4 L • 22.4 L is called molar volume • Calculate the volume at STP • Volume of gas = mole of gas given X 22.4 L /1 mol

  15. Calculate moles from L • Moles = volume of gas given X 1 mole / 22.4 L

  16. Calculate molar mass from density • Molar mass = density at STP X molar volume at STP • g/mole = gram/L X 22.4 L / 1 mole

  17. 10.3 Percent Composition and Chemical Formulas • After completing this section, you will be able to: • Describe how to calculate percent by mass of an element in a compound • Interpret an empirical formula • Distinguish between empirical and molecular formulas

  18. Percent composition is the relative amounts of the elements in a compound • AKA percent by mass of each element in the compound • % mass of element = mass of element/ mass of compound X 100

  19. Percent composition from the chemical formula • % by mass = mass of element in 1 mol compound/ molar mass of compound X 100

  20. Percent composition may be used as a conversion factor • Ex: if you have 82 g of propane (C3H8), how many grams of C and H are present if propane is 81.8% C and 18% H?

  21. Empirical formula is the lowest whole-number ratio of atoms of the elements in a compound • Called the basic ratio • May not be the same as the molecular formula • Ex: hydrogen peroxide is H2O2, the empirical formula would be HO

  22. Molecular formula shows the actual number of each atom in a molecule of the compound • May be the same as the empirical formula or it may be a simple whole-number multiple of the empirical formula • Need to know the molar mass in order to determine the molecular formula

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