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SKILLS Project Calculating Molar Masses

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What are molar masses? A molar mass is the weight, in grams, of a mole of any element or compound. Each element on the periodic table is assigned a molar mass. A compound’s molar mass is the sum of the masses of each element it contains.

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Connections Remember, a mole is 6.022e23 atoms/molecules of a substance. So, the molar mass is a measurement of the total mass of a mole of these particles. Due to the fact that the atoms of each element have different atomic masses, their molar masses are also different.

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To find molar mass: List the elements in the compound. Find the amount of each element. Multiply each element by their unique molar masses. Add up the individual totals to find the total molar mass of the compound.

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Example 1: Mass of Iron A single element’s molar mass can be determined directly from the periodic table. To do so, look at the atomic mass at the bottom of an element’s symbol. This number, in grams, is the molar mass of a single element. We typically round the molar mass of an element to the nearest hundredth. So, 1 mol Fe = 55.85 g Fe. The molar mass of Fe is 55.85 g/mol 1 mol Fe = 55.85 g Fe

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Example 2: Fe 2 O 3 1.List the elements in the compound.2.Find the amount of each element.3.Multiply each element by their unique molar masses 4.Add up the individual totals to find the total molar mass of the compound. Fe O X X 2 3 = = 55.85 g 16.00 g X X 2 3 = = 111.70 g 48.00 g = 159.70 g/mol 1 mol Fe 2 O 3 = 159.70 g

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Example 3: N 2 O 4 1.List the elements in the compound.2.Find the amount of each element.3.Multiply each element by their unique molar masses 4.Add up the individual totals to find the total molar mass of the compound. N O X X 2 4 = = 14.01 g 16.00 g X X 2 4 = = 28.02 g 64.00 g = 92.02 g/mol 1 mol N 2 O 4 = 92.02 g

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Example 4: Na 3 PO 4 1.List the elements in the compound.2.Find the amount of each element.3.Multiply each element by their unique molar masses 4.Add up the individual totals to find the total molar mass of the compound. Na P X X 3 1 = = 22.99 g 30.97 g X X 3 1 = = 68.97 g 30.97 g = 163.94 g/mol 1 mol Na 3 PO 4 = 163.94 g O X 4 = 16.00 g X 4 = 64.00 g

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Example 5: LiC 2 H 3 O 2 1.List the elements in the compound.2.Find the amount of each element.3.Multiply each element by their unique molar masses 4.Add up the individual totals to find the total molar mass of the compound. Li C X X 1 2 = = 6.94 g 12.01 g X X 1 2 = = 6.94 g 24.02 g = 65.99 g/mol 1 mol LiC 2 H 3 O 2 = 65.99 g H X 3 = 1.01 g X 3 = 3.03 g O X 2 = 16.00 g X 2 = 32.00 g

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Note on Parentheses Many compounds contain polyatomics listed in parentheses, such as: –Fe 2 (SO 4 ) 3 The subscript outside the parenthesis will multiply all of the substances inside. –Fe x 2 –(S x 1) x 3 = S x 3 –(O x 4) x 3 = O x 12

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Example 6: Ir 2 (CO 3 ) 3 1.List the elements in the compound.2.Find the amount of each element.3.Multiply each element by their unique molar masses 4.Add up the individual totals to find the total molar mass of the compound. Ir C X X 2 3 = = 192.22 g 12.01 g X X 2 3 = = 384.44 g 36.03 g = 564.47 g/mol 1 mol Ir 2 (CO 3 ) 3 = 564.47 g O X 9 = 16.00 g X 9 = 144.00 g

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Example 7: (NH 4 ) 3 P 1.List the elements in the compound.2.Find the amount of each element.3.Multiply each element by their unique molar masses 4.Add up the individual totals to find the total molar mass of the compound. N H X X 4 12 = = 14.01 g 1.01 g X X 4 12 = = 56.04 g 12.12 g = 99.13 g/mol 1 mol (NH 4 ) 3 P = 99.13 g P X 1 = 30.97 g X 1 =

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Example 8: (NH 4 ) 2 Cr 2 O 7 1.List the elements in the compound.2.Find the amount of each element.3.Multiply each element by their unique molar masses 4.Add up the individual totals to find the total molar mass of the compound. N H X X 2 8 = = 14.01 g 1.01 g X X 2 8 = = 28.02 g 8.08 g = 304.10 g/mol 1 mol (NH 4 ) 2 Cr 2 O 7 = 304.10 g Cr X 2 = 52.00 g X 3 = 156.00 g O X 7 = 16.00 g X 7 = 112.00 g

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Example 9: AgCN 1.List the elements in the compound.2.Find the amount of each element.3.Multiply each element by their unique molar masses 4.Add up the individual totals to find the total molar mass of the compound. Ag C X X 1 1 = = 107.87 g 12.01 g X X 1 1 = = 107.87 g 12.01 g = 133.89 g/mol 1 mol AgCN = 133.89 g N X 1 = 14.01 g X 1 =

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Example 10: V(NO 2 ) 5 1.List the elements in the compound.2.Find the amount of each element.3.Multiply each element by their unique molar masses 4.Add up the individual totals to find the total molar mass of the compound. V N X X 1 5 = = 50.94 g 14.01 g X X 1 5 = = 50.94 g 70.05 g = 280.99 g/mol 1 mol V(NO 2 ) 5 = 280.99 g O X 10 = 16.00 g X 10 = 160.00 g

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Example 11: Ba(OH) 2 1.List the elements in the compound.2.Find the amount of each element.3.Multiply each element by their unique molar masses 4.Add up the individual totals to find the total molar mass of the compound. Ba O X X 1 2 = = 137.33 g 16.00 g X X 1 2 = = 137.33 g 32.00 g = 171.35 g/mol 1 mol Ba(OH) 2 = 171.35 g H X 2 = 1.01 g X 2 = 2.02 g

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Example 12: Au(NO 3 ) 4 1.List the elements in the compound.2.Find the amount of each element.3.Multiply each element by their unique molar masses 4.Add up the individual totals to find the total molar mass of the compound. Au N X X 1 4 = = 196.97 g 14.01 g X X 1 4 = = 196.97 g 56.04 g = 445.01 g/mol 1 mol Au(NO 3 ) 4 = 445.01 g O X 12 = 16.00 g X 12 = 192.00 g

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Practice on Your Own: Determine the molar masses of the following compounds: –CuCl 2 – –O 2 – –Se 2 N 3 – –CCl 4 – –Zr 3 (PO 4 ) 4 – 134.55 g/mol 32.00 g/mol 199.95 g/mol 153.81 g/mol 653.55 g/mol

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Practice on Your Own (2): Determine the molar masses of the following compounds: 6.Pt(C 2 H 3 O 2 ) 7 – 7.SO 2 – 8.ArF 2 – 9.Co(ClO 4 ) 2 – 10. (NH 4 ) 2 S 2 O 3 – 608.43 g/mol 64.07 g/mol 77.95 g/mol 258.83 g/mol 148.24 g/mol

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Congratulations! You have mastered the ability to find the molar mass of a substance based on its formula. This skill is incredibly useful and allows you to find the conversion factors to turn measurements you take in lab, i.e. grams, into moles for chemical reactions.

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