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Understanding Molar Mass in Chemistry: A Practical Guide

Explore the relationship between atomic mass and moles, learn to calculate moles from element mass and vice versa, and understand the concept of molar mass. Practice problems included.

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Understanding Molar Mass in Chemistry: A Practical Guide

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  1. Mass and the Mole Chemistry Section 11.2

  2. Objectives • Relate the mass of an atom to the mass of a mole of atoms. • Calculate the number of moles in a given mass of an element and the mass of a given number of moles of an element. • Calculate the number of moles of an element when given the number of atoms of the element. • Calculate the number of atoms of an element when given the number of moles of the element.

  3. The Mass of the Mole • Do a dozen limes have the same mass as a dozen eggs? • No. • The same is true for moles. • One mol of Fe does not have the same mass as one mol of Ag.

  4. Molar Mass • This is IMPORTANT! • Mass in grams of 1 mole of any pure substance is termed the molar mass. • Molar mass of any element equals its atomic mass • Units are g/mol

  5. Molar Mass • For Example: • Manganese • What is Manganese’s molar mass? • 54.94 amu OR g/mol • What does one mole of Manganese weigh? • 54.94 g • How many atoms of Manganese are in one mol? • 6.022 x 1023

  6. Molar Mass • We can use the molar mass of every element as a conversion factor. • Example: # of mol x # of grams = mass 1 mol

  7. Mole to Mass Conversions • Chromium (Cr) is a transition element. • Calculate the mass in grams of 0.0450 moles of Cr. • Always start with ? • What is given in the problem! • 0.0450 mol Cr

  8. Mole to Mass Conversions • Molar mass is the conversion factor • 52.00 g/1 mol Cr • Put what you want to end up with on the top of the conversion factor.

  9. Mole to Mass Conversions • 0.0450 mol Cr x 52 g = 2.34 g Cr 1 mol

  10. Practice Problems • Pg. 316 • 12

  11. Mass to Atoms Conversions • Let’s take the conversion factor further. • 2 step conversion factor problems Representative Particles Mass Moles

  12. Mass to Atoms Conversions • If given the mass of an element, you can find the number of atoms in that mass. • Example: How many atoms of gold are in a pure gold nugget having a mass of 25.0 g?

  13. Mass to Atoms Conversions • Always start with? • What is given in the problem. • 25.0 g Au • Mass  Moles  Particles

  14. Mass  Moles • Use molar mass of Au conversion factor: 196.97 g 1 mol Au

  15. Moles  Particles • Avogadro’s conversion: 6.022 x 1023 atoms 1 mol Au

  16. MassMoles Atoms • 25.0 g Au x 1 mol Au x 6.022 x 1023atoms = 196.97g Au 1 mol Au 7.65 x 1022 atoms Au 1st conversion 2nd conversion

  17. Practice Problems • Pg 318 • 14

  18. Homework • Molar mass worksheet

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