# 1 Section 3.3 The Mole: A Measurement of Matter n OBJECTIVES: –Describe how Avogadro’s number is related to a mole of any substance.

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1 Section 3.3 The Mole: A Measurement of Matter n OBJECTIVES: –Describe how Avogadro’s number is related to a mole of any substance.

2 Section 3.3 The Mole: A Measurement of Matter n OBJECTIVES: –Calculate the mass of a mole of any substance.

3 What is a Mole? What is a Mole? n You can measure mass, n or volume, n or you can count pieces. n We measure mass in grams. n We measure volume in liters. n We count pieces in MOLES.

4 Moles (abbreviated: mol) n Defined as the number of carbon atoms in exactly 12 grams of carbon-12. n 1 mole is 6.02 x 10 23 particles. n Treat it like a very large dozen n 6.02 x 10 23 is called Avogadro’s number.

5 Representative particles n The smallest pieces of a substance. –For a molecular compound: it is the molecule. –For an ionic compound: it is the formula unit. –For an element: it is the atom.

6 Types of questions n How many molecules of CO 2 are there in 4.56 moles of CO 2 ? n How many moles of water is 5.87 x 10 22 molecules? n How many atoms of carbon are there in 1.23 moles of C 6 H 12 O 6 ? n How many moles is 7.78 x 10 24 formula units of MgCl 2 ?

7 Measuring Moles n Remember relative atomic mass? n The amu was one twelfth the mass of a carbon-12 atom. n Since the mole is the number of atoms in 12 grams of carbon-12, n the decimal number on the periodic table is also the mass of 1 mole of those atoms in grams.

8 Gram Atomic Mass (gam) n Equals the mass of 1 mole of an element in grams n 12 grams of C has the same number of pieces as 1 gram of H and 56 grams of iron. n We can write this as 12 g C = 1 mole C n We can count things by weighing them.

9 Examples n How much would 2.34 moles of carbon weigh? n How many moles of magnesium is 24 g of Mg? n How many atoms of lithium is 1.00 g of Li? n How much would 3.45 x 10 22 atoms of U weigh?

10 What about compounds? n in 1 mole of H 2 O molecules there are two moles of H atoms and 1 mole of O atoms n To find the mass of one mole of a compound –determine the moles of the elements they have –Find out how much they would weigh –add them up

11 What about compounds? n What is the mass of one mole of CH 4 ? 1 mole of C = 12 g 4 mole of H x 1 g = 4 g 1 mole CH 4 = 12 + 4 = 16 g n The Gram Molecular Mass (gmm) of CH 4 is 16 g –this is the mass of one mole of a molecular compound.

12 Gram Formula Mass (gfm) n The mass of one mole of an ionic compound. n Calculated the same way as gmm. n What is the GFM of Fe 2 O 3 ? 2 moles of Fe x 56 g = 112 g 3 moles of O x 16 g = 48 g The GFM = 112 g + 48 g = 160 g

13 Section 3.3 Mole-Mass and Mole-Volume Relationships n OBJECTIVES: –Use the molar mass to convert between mass and moles of a substance.

14 Section 3.3 Mole-Mass and Mole-Volume Relationships n OBJECTIVES: –Use the mole to convert among measurements of mass, volume, and number of particles.

15 Molar Mass n Molar mass is the generic term for the mass of one mole of any substance (in grams) n The same as: 1) gram molecular mass, 2) gram formula mass, and 3) gram atomic mass- just a much broader term.

16 Examples n Calculate the molar mass of the following and tell what type it is: n Na 2 S nN2O4nN2O4nN2O4nN2O4 nCnCnCnC n Ca(NO 3 ) 2 n C 6 H 12 O 6 n (NH 4 ) 3 PO 4

17 Molar Mass n The number of grams of 1 mole of atoms, ions, or molecules. n We can make conversion factors from these. –To change grams of a compound to moles of a compound.

18 For example n How many moles is 5.69 g of NaOH?

19 For example n How many moles is 5.69 g of NaOH?

20 For example n How many moles is 5.69 g of NaOH? l need to change grams to moles

21 For example n How many moles is 5.69 g of NaOH? l need to change grams to moles l for NaOH

22 For example n How many moles is 5.69 g of NaOH? l need to change grams to moles l for NaOH l 1mole Na = 23 g 1 mol O = 16 g l 1 mole of H = 1 g

23 For example n How many moles is 5.69 g of NaOH? l need to change grams to moles l for NaOH l 1mole Na = 23 g 1 mol O = 16 g l 1 mole of H = 1 g l 1 mole NaOH = 40 g

24 For example n How many moles is 5.69 g of NaOH? l need to change grams to moles l for NaOH l 1mole Na = 23 g 1 mol O = 16 g l 1 mole of H = 1 g l 1 mole NaOH = 40 g

25 For example n How many moles is 5.69 g of NaOH? l need to change grams to moles l for NaOH l 1mole Na = 23 g 1 mol O = 16 g l 1 mole of H = 1 g l 1 mole NaOH = 40 g

26 Examples n How many moles is 4.56 g of CO 2 ? n How many grams is 9.87 moles of H 2 O? n How many molecules is 6.8 g of CH 4 ? n 49 molecules of C 6 H 12 O 6 weighs how much?

27 Gases n Many of the chemicals we deal with are gases. –They are difficult to weigh. n Need to know how many moles of gas we have. n Two things effect the volume of a gas –Temperature and pressure n We need to compare them at the same temperature and pressure.

28 Standard Temperature and Pressure n 0ºC and 1 atm pressure n abbreviated STP n At STP 1 mole of gas occupies 22.4 L n Called the molar volume n 1 mole = 22.4 L of any gas at STP

29 Examples n What is the volume of 4.59 mole of CO 2 gas at STP? n How many moles is 5.67 L of O 2 at STP? n What is the volume of 8.8 g of CH 4 gas at STP?

30 Density of a gas n D = m / V –for a gas the units will be g / L n We can determine the density of any gas at STP if we know its formula. n To find the density we need the mass and the volume. n If you assume you have 1 mole, then the mass is the molar mass (from PT) n At STP the volume is 22.4 L.

31 Examples n Find the density of CO 2 at STP. n Find the density of CH 4 at STP.

32 The other way n Given the density, we can find the molar mass of the gas. n Again, pretend you have 1 mole at STP, so V = 22.4 L. n m = D x V n m is the mass of 1 mole, since you have 22.4 L of the stuff. n What is the molar mass of a gas with a density of 1.964 g/L? n 2.86 g/L?

33 Summary n These four items are all equal: a) 1 mole b) molar mass (in grams) c) 6.02 x 10 23 representative particles d) 22.4 L at STP Thus, we can make conversion factors from them.

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