A dozen. Molar Mass. Mass of atoms

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1 A dozen Molar Mass Science 10 is a number of objects. A dozen eggs, a dozen cars, and a dozen people are all 12 objects. But a dozen cars has a much greater mass than a dozen eggs because the mass of each car is much greater than the mass of each egg. 1 2 Mass of atoms An Au atom contains 79 p + and 118 n. An H atom contains one p +. The mass of an Au atom is about 197 times greater than the mass of an H atom. A dozen A dozen Au atoms have a mass that is about 197 times greater than the mass of a dozen H atoms. But atoms are very small A gold atom has a mass of about 3 x g or g 3 4

2 The mole Because atoms are so small it is not practical to talk about individual atoms. Chemists talk about moles of atoms. Like a dozen, a mole is a number of atoms. One mole of atoms is about 6.02 x atoms. Avogadro s number Just like 12 eggs is called a dozen eggs, 6.02!x!10 23 atoms is a mole of atoms x is known as Avogadro s number. 5 6 Power of the mole A mole of Au has 6.02 x atoms. A mole of Mg has 6.02 x atoms. The mole allows easy comparisons of amounts of atoms in the same way that the dozen allows easy comparisons of amounts of eggs. Why 6.02 x 10 23? An oxygen atom has 8 p + and 8 n. Its atomic mass number is = x oxygen atoms have a mass of 16!grams. 16 grams of oxygen atoms is a convenient amount of oxygen, whereas 16 atoms is impractical. 7 8

3 Molar mass is the mass of one mole of a substance. The atomic mass numbers of an element on the periodic table is the average mass, in grams, of one mole of that element s atoms. Example 1 What is the molar mass of Zn? g/mol 9 10 Example 2 What is the molar mass of methane gas? Methane is CH4, meaning each molecule has one C atom and 4 H atoms. Each C: g/mol Each H: 1.01 g/mol Total: g/mol + (4 x 1.01 g/mol) = g/mol Example 3 What is the molar mass of water? H2O: H: 2 x 1.01 g/mol = 2.02 g/mol O: 1 x g/mol = g/mol M = g/mol 11 12

4 Example 4 What is the molar mass of iron (III) oxide? Your turn Page 108 practice questions Fe2O3: Fe: 2 x g/mol = g/mol O: 3 x g/mol = g/mol M = g/mol Number of moles One mole of Cu has a mass of g. The molar mass (M) for Cu is g/mol g of Cu would be composed of 6.02!x!10 23 atoms g of Cu would be two moles of Cu, or 2 x 6.02 x = x atoms. Number of moles We can find the number of moles in a sample of any substance if we know (or can determine) the substance s molar mass, and we know the mass of the sample. We use the molar mass formula 15 16

5 Number of moles mass n= m (g) M Example 1 Find the number of moles in 2.00!g of helium. Find the molar mass: number of moles (mol) molar mass (g/mol) He: 1 x 4.00 g/mol = 4.00 g/mol M = 4.00 g/mol Example 1 n= m M = 2.00!g 4.00!g/mol =!0.5!mol Example 1 What does 0.5 mol mean? one mol = 6.02 x atoms 0.5 mol = 0.5 x 6.02 x atoms 0.5 mol = 3.01 x atoms 19 20

6 Example 2 Find the number of moles in 6.00 g of strontium chloride. Find the molar mass: SrCl2: Sr: 1 x g/mol = g/mol Cl: 2 x g/mol = g/mol M = g/mol Example 2 n= m M = 6.00!g !g/mol =! !mol =!0.0379!mol Significant Digits are a way of representing how accurate a measurement is. SD Rules The digits 1 through 9 are always significant. Leading zeros are not significant. All other zeros are significant. In scientific notation, the digits before the x!10 are significant. Exact numbers have unlimited significant digits

7 SD examples Examples of 2 SD x Exact Numbers These are exact numbers: 15 students $5.25 SD Rules Part 2 In multiplication or division calculations, the answer should be rounded to the least number of SD from the numbers used

8 SD in Example n= m M = 6.00!g !g/mol =! !mol =!0.0379!mol 3 SD 3 SD Example 2 Find the number of moles in 9.50!g of ethanol. C2H5OH: C: 2 x g/mol = g/mol H: 6 x 1.01 g/mol = 6.06 g/mol O: 1 x g/mol = g/mol M = g/mol Example 2 Find the number of moles in 9.50!g of ethanol. n= m M = 9.50!g 46.08!g/mol = !mol =0.206!mol Example 3 Find the number of moles in 1.1!kg of gold (II) phosphate. Au3(PO4)2: Au: 3 x g/mol = g/mol P: 2 x g/mol = g/mol O: 8 x g/mol = g/mol M = g/mol 31 32

9 Example 3 mol to mass Find the number of moles in 1.1!kg of gold (II) phosphate. n= m M = 1100!g !g/mol = !mol m=nm =1.4!mol Example 4 Find the mass of mol of sodium carbonate. Na2CO3: Na: 2 x g/mol = g/mol C: 1 x g/mol = g/mol O: 3 x g/mol = g/mol M = g/mol Example 4 Find the mass of mol of sodium carbonate. m=nm =0.205!mol!x!105.99!g/mol = !g =!21.7!g 35 36

10 Example 5 Find the mass of mol of calcium chloride. CaCl2: Ca: 1 x g/mol = g/mol Cl: 2 x g/mol = g/mol M = g/mol Example 5 Find the mass of mol of calcium chloride. m=nm =0.015!mol!x!110.98!g/mol =!1.6647!g =!1.7!g 37 38

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