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Chemical Quantities

Learn how to measure matter, count atoms, convert moles, calculate molar mass, determine percent composition, and find empirical formulas.

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Chemical Quantities

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  1. Chemical Quantities Avogadro’s Number

  2. Many words are used to express a specific quantity. • Pair =2 • Dozen = 12 How do we measure matter? • By mass – 1lb of bananas • By volume – a gallon of milk • By count – 20 calculators

  3. How do we measure matter? What if I wanted to know how many ATOMS were in this copper penny? Would you like to count? Could you? A mole is the most appropriate unit when dealing with atoms.

  4. Avogadro’s Number You can count particles if you introduce a term that represents a specified number of particles. 1 mole = 6.02 x 1023 (Avogadro’s number) representative particles. Or 602,000,000,000,000,000,000,000 A very large number!

  5. MOLE A mole is the SI unit that measures the amount of a substance. A mole can be related to: The number of particles The mass The volume of elements and compounds

  6. HOW LARGE IS IT??? • 1 mole of hockey pucks would equal the mass of the moon! • 1 mole of basketballs would fill a bag the size of the earth! • 1 mole of pennies would cover the Earth 1/4 mile deep!

  7. MOLE Use dimensional analysis to convert: moles to molecules moles to atoms moles to formula units Conversion Factor to Remember: 1 mol = 6.02 x 1023 The units for Avogadro’s number will be atoms, molecules, or formula units. The unit depends on the type of compound or if just a singular element.

  8. MOLE Examples How many moles are in 3.25 X 1020 atoms of lead? How many moles are there in 8.3 x 1026 molecules? How many atoms are in 0.425 mol nitrogen? How many formula units are in 8.56 X 10-3 mol NaCl?

  9. Chemical Quantities Molar Mass

  10. Molar Mass Molar Mass equals 1 mole of an element or compound. To calculate Molar Mass: Obtain all of the masses of the involved elements from the periodic table. Round mass to 2 decimal places. Multiply each element’s mass by the subscript. Add these masses Round the answer to the proper number of significant digits.

  11. Terms to Know for Mass of a Compound Molecular mass - mass of one molecule of a covalent compound (unit = amu) Formula mass - mass of one formula unit of an ionic compound (unit = amu) Molar mass - mass of one mole of anything: molecules, formula units, atoms, etc. (unit = g/mol).

  12. Difference between Molecular Mass & Molar Mass Same numerical value, but different units Molar Mass of Element Examples • Carbon = 12.01 g/mol • Aluminum = 26.98 g/mol

  13. VIDEO to more Discussion on the MOLE The Mole and Avogadro's Number from Khan Academy

  14. Molar Mass of Compounds Examples • Water H2O = 2 hydrogen & 1 oxygen 2(1.01) + 16.00 = 18.02 g/mol • Sucrose C12H22O11 = 12 carbon, 22 hydrogen, 11 oxygen 12(12.01) + 22(1.01) + 11(16.00) = 342.34 g/mol

  15. Molar Mass of Compounds Examples NaCl 1 sodium, 1 chlorine 1(22.99) + 1(35.45) = 58.44 g/mol Sodium Chloride • Find the formula • How many of each • Multiply masses by subscripts and add the masses

  16. Moles to Mass • Use dimensional analysis to convert • moles to mass or mass to moles. • Mass unit = grams • Conversion Factor to Remember: • 1 mol = molar mass of the element or compound

  17. Moles to Mass Examples • How many moles of carbon are in 26 g of carbon? • How many grams are there in 2.37 moles of CO2?

  18. Moles to Mass Examples • How many moles are present in 142.1 grams of NaCl? • How many grams are in 3.34 moles of potassium bromide?

  19. Mole Calculations Learned MASS IN GRAMS MOLES NUMBER OF PARTICLES/ ATOMS LITERS GAS X X

  20. Chemical Quantities Percent Composition

  21. Percent Composition Percent composition is the percent of that element in the compound….the total percent for the compound will equal 100%. Formula for Percent Composition:

  22. Percent Composition Examples Example: 9.03 g Mg combined completely with 3.48 g N to form a compound. What is the percent composition of the compound? 9.03g + 3.48g = 12.51 g (total mass) 9.03/12.51 x 100 = 72.2 % of Mg 3.48g/12.51 x 100 = 27.8% of N Check: Add % Composition of each element to see if adds up to 100% 72.2 % of Mg + 27.8% of N = 100%

  23. Percent Composition Examples When the mass of each element is not given, calculate the molar mass of each element in the compound before calculating the % composition. Example: Calculate the percent composition of C2H6. Mass of 1 mol of Carbon = 12.0g x 2 = 24g Mass of 1 mol of Hydrogen = 1.0g x 6 = 6g 24g/30g x 100 = 80% 6g/30g x 100 = 20 % Be sure to check your work.

  24. More Percent Composition Examples • Calculate the % Composition of these compounds: • NaHSO4 • NH4Cl

  25. Chemical Quantities Empirical Formulas

  26. Empirical Formulas The empirical formula gives the lowest whole number ratio of the elements in a compound. Reduces the subscripts. The empirical is sometimes and sometimes not the same as the molecular or “actual” formula. For ionic compounds, the formula unit is the empirical formula. Example when NOT: The empirical formula for H2O2 is HO.

  27. Empirical Formulas The molecular formula for vitamin C is C6H8O6. What is the empirical formula? A. CHO B. C3H4O3 C. CH2O D. C2H4O2

  28. Empirical Formulas Which of the following is an empirical formula? A. C3H6 B. NaCl C. CH4 D. C4H10

  29. VIDEO to more Discussion on the MOLE Emp and Mol formulas from Percent composition

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