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Understanding the Mole: Molar Mass, Avogadro's Number & Conversions

Learn about the mole as a unit to measure small objects, Avogadro's number, molar mass, converting moles to particles, and more in chemistry. Understand the difference between atomic mass and formula mass and how to calculate them.

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Understanding the Mole: Molar Mass, Avogadro's Number & Conversions

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  1. The Mole (no, it’s not an animal!)

  2. The Mole • It is a quantity, like a dozen, a trio, a pair • It is used to measure a quantity of very small objects, i.e., atoms and molecules • It is 6.02 x 1023, also known as Avogadro’s number • Not useful for measuring everyday items

  3. How Much is a Mole? • 6.02 X 1023 Watermelon Seeds: Would be found inside a melon slightly larger than the moon. • 6.02 X 1023 Donut Holes: Would cover the earth and be 5 miles (8 km) deep. • 6.02 X 1023  Pennies: Would make at least 7 stacks that would reach the moon. • 6.02 X 1023  Grains of Sand: Would be more than all of the sand on Miami Beach. • 6.02 X 1023  Blood Cells: Would be more than the total number of blood cells found in every human on earth. • 6.02 X 1023  C02 Molecules: Would be found in 1 Mole BalloonTM at STP.

  4. Why did scientists need a mole? • They knew that equal volumes of gases at standard temp and pressure (STP) had an equal number of particles • They knew that 32 g of O2 and 2 g of H2 had a volume of 22.4 L. • They determined that that number of particles was 6.02 x 1023, or one mole

  5. What is the difference between atomic mass and formula mass? • Atomic mass is expressed in atomic mass units • Atomic mass unit: (amu) one twelfth the mass of the carbon 12-isotope • Most elements do not have whole number mass • the atomic mass is an average of the masses of all of the isotopes of that element • the actual mass of a proton and neutron is not equal to exactly 1 amu • Formula mass: the sum of the atomic masses of all of the atoms in a compound in amus

  6. How do I calculate formula mass? • It is found by summing the atomic masses of all of the atoms in a compound • The mass unit is amus • Example: H2SO4 has 2 H’s, 1 S, and 4 O’s 2 (1.01 amu) + (32.07 amu) + 4(16.00 amu) = 98.09 amu So, the formula mass of H2SO4 is 98.09 amu

  7. What is molar mass? • Mass of 1 mole of a compound • Measured in grams • It is the mass in grams of one mole of a given substance • It is equal to the average atomic mass of that element, written in grams • It depends on the masses of the elements that make up the substance • It is calculated exactly like formula mass, but with different units (g instead of amu)

  8. How to find molar mass • Find the molar mass of Na2SO4 • Find the molar mass of MgSO4· 7H2O

  9. How do I convert from moles to mass? • You must use molar mass • How many moles of water are in 18.01 g of water? • What is the mass of 15.2 moles of CO2?

  10. What is Avogadro’s Number? • 6.02 x 1023: the number of representative particles (atoms, molecules, formula units) in exactly one mole of a substance • Atoms – The representative particle in an element • Molecules – The representative particle in a molecule of liquid • Formula Units – The representative particle in a compound • Mole: the fundamental SI unit used to measure the amount of a substance

  11. How can I convert moles to number of particles? • Avogadro’s Number is used as a factor in converting from a given number of moles to the equivalent number of atoms. • To convert from moles to number of particles, you must use dimensional analysis.

  12. How do I convert moles to particles? • Remember Avogadro’s Number • 6.02 x 1023 particles 1 mole • So, # moles x 6.02 x 1023 particles = 1 mole # particles

  13. Sample Problems A) # atoms in 2.50 mol Zn B) # molecules in 11.5 mol H2O

  14. How do I convert moles to volume? • Molar volume: the volume of 1 mole of a gas at standard temperature and pressure (0°C and 1 atmosphere; abbreviated STP) • Gases change volume with changes in temperature, pressure, or both; so, temp and press must be specified when giving the volume of a gas • 1 mole of gas = 22.4 L • This is true ONLY at STP

  15. To convert moles to volume • How many moles will be in 44.8 L of O2? • What is the volume of 0.725 moles of NH3?

  16. How do I combine multiple conversions? • You can combine the above processes together to simplify calculations

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