# MOLECULAR MASS AND FORMULA MASS

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1 1 MOLECULAR MASS AND FORMULA MASS Molecular mass = sum of the atomic weights of all atoms in the molecule. Formula mass = sum of the atomic weights of all atoms in the formula unit.

2 2 MOLECULAR MASS AND FORMULA MASS Ammonium sulfate, (NH 4 ) 2 SO 4 Magnesium perchlorate, Mg(ClO 4 ) 2 Carbon tetrachloride, CCl 4 Diphosphorus pentoxide P 2 O 5

3 3 Fig. 7-1, p. 176

4 4 Counting Atoms Chemistry is a quantitative science we need a counting unit. MOLE 1 mole is the amount of substance that contains as many particles (atoms, molecules) as there are in 12.0 g of 12 C. 518 g of Pb, 2.50 mol

5 5 Particles in a Mole Avogadro s Number Amedeo Avogadro x There is Avogadro s number of particles in a mole of any substance.

6 6 MOLECULAR MASS AND MOLAR MASS Molecular mass = sum of the atomic weights of all atoms in the molecule. Molar mass = molecular weight in grams

7 Molar Mass 7 1 mol of 12 C = g of C = x atoms of C g of 12 C is its MOLAR MASS Taking into account all of the isotopes of C, the molar mass of C is g/mol

8 8 One-mole Amounts

9 PROBLEM: What amount of Mg is represented by g? How many atoms? Mg has a molar mass of g/mol. 9 How many atoms in this piece of Mg? = 4.95 x atoms Mg

10 What is the molar 10 mass of ethanol, C 2 H 6 O? 1 mol contains 2 mol C (12.01 g C/1 mol) = g C 6 mol H (1.01 g H/1 mol) = 6.06 g H 1 mol O (16.00 g O/1 mol) = g O TOTAL = molar mass = g/mol

11 11 Tylenol Formula = Molar mass = C 8 H 9 NO g/mol

12 Molar Mass 12 Note that the mass of water is included in the molar mass of a compound.

13 13 Percent Composition Percent composition is the percentage by mass of each element in the compound. %A = mass of A X 100 mass of compound Sum of percentages should equal 100

14 14 Percent Composition A pure compound always consists of the same elements combined in the same proportions by weight. Therefore, we can express molecular composition as PERCENT BY WEIGHT Ethanol, C 2 H 6 O 52.13% C 13.15% H 34.72% O

15 15 Percent Composition Consider NO 2, Molar mass =? What is the weight percent of N and of O? What are the weight percentages of N and O in NO?

16 16 Determining Formulas In chemical analysis we determine the % by weight of each element in a given amount of pure compound and derive the EMPIRICAL or SIMPLEST formula. PROBLEM: A compound of B and H is 81.10% B. What is its empirical formula?

17 17 A compound of B and H is 81.10% B. What is Because it contains only B and H, it must contain 18.90% H. In g of the compound there are g of B and g of H. Calculate the number of moles of each constituent.

18 18 A compound of B and H is 81.10% B. What is Calculate the number of moles of each element in g of sample.

19 19 A compound of B and H is 81.10% B. What is Take the ratio of moles of B and H. Always divide by the smaller number. But we need a whole number ratio. 2.5 mol H/1.0 mol B = 5 mol H to 2 mol B EMPIRICAL FORMULA = B 2 H 5

20 A compound of B and H is 81.10% B. Its empirical formula is B 2 H 5. What is Is the molecular formula B 2 H 5, B 4 H 10, B 6 H 15, B 8 H 20, etc.? 20 B 2 H 6 B 2 H 6 is one example of this class of compounds.

21 21 A compound of B and H is 81.10% B. Its empirical We need to do an EXPERIMENT to find the MOLAR MASS. Here experiment gives 53.3 g/mol Compare with the mass of B 2 H 5 = g/unit Find the ratio of these masses. Molecular formula = B 4 H 10

22 The Empirical Formula A sample is made up of 1.61 g P and 2.98 g F. Find the EF. Determine the masses of each element: 1.61 g P 2.98 g F Convert the masses into moles of each element: 1.61 g P x 1 mol P = mol P g P 2.98 g F x 1 mol F = mol F g F Express the moles as the smallest possible ratio: = = Write the EF using the values above as subscripts: 22 PF 3

23 The Empirical Formula A compound is found to contain 20.0 % carbon, 2.2 % hydrogen and 77.8 % chlorine. Determine the EF. Determine the masses of each element assuming 100 g: 20.0 g C 2.2 g H 77.8 g Cl Convert the masses into moles of each element: 20.0 g C x 1 mol C = 1.67 mol C g C 2.2 g H x 1 mol H = 2.18 mol H g H 77.8 g Clx 1 mol Cl = 2.19 mol Cl g Cl Express the moles as the smallest possible ratio: 1.67= = = Write the EF using the values above as subscripts: 23 CH 1.3 Cl 1.3 = C 3 H 4 Cl 4 Must have a whole number ratio!

24 The Molecular Formula Fructose is found to contain 40.0 % carbon, 6.71 % hydrogen and the rest oxygen. The molar mass of fructose is g/mol. Determine the EF and molecular formula. Determine the masses of each element assuming 100 g: 40.0 g C 6.71 g H g O Convert the masses into moles of each element: 40.0 g C x 1 mol C = 3.33 mol C g C 6.71 g H x 1 mol H = 6.66 mol H g H g Ox 1 mol O = 3.33 mol O g O Express the moles as the smallest possible ratio: CH 2 O 24

25 The Molecular Formula Fructose is found to contain 40.0 % carbon, 6.71 % hydrogen and the rest oxygen. The molar mass of fructose is g/mol. Determine the EF and molecular formula. Determine the mass of the EF (CH 2 O): g/mol + 2(1.008 g/mol) + 16 g/mol = g/mol Determine the number of EF units in the molecule: 25 Molar mass compound= g/mol Molar mass EF g/mol = 6 Write the molecular formula: (CH 2 O) 6 = C 6 H 12 O 6

26 26 DETERMINE THE FORMULA OF A COMPOUND OF Sn AND I Sn(s) + some I 2 (s) f SnI x

27 27 Data to Determine the formula of a Sn I Compound Reaction of Sn and I 2 is done using excess Sn. Mass of Sn in the beginning = g Mass of iodine (I 2 ) used = g Mass of Sn remaining = g

28 28 Tin and Iodine Compound Find the mass of Sn that combined with g I 2. Mass of Sn initially = g Mass of Sn recovered = g Mass of Sn used = g Find moles of Sn used:

29 29 Tin and Iodine Compound Now find the number of moles of I 2 that combined with 3.83 x 10-3 mol Sn. Mass of I 2 used was g. How many mol of iodine atoms? = x 10-2 mol I atoms

30 30 Tin and Iodine Compound Now find the ratio of number of moles of moles of I and Sn that combined. Empirical formula is SnI 4

31 Percent Composition If you know the percent composition of a compound, you can find the amount of any element in a known amount of the compound. 31 How many g of potassium are in g of K 2 S? (MW = g/mol) Method 1: Method 2: g K 2 S x 78.2 g K = g K g K 2 S Use the percent composition as a conversion factor, g K/100 g K 2 S g K 2 S x g K 100 g K 2 S = g K Both methods are equally valid!

32 Avogadro s Number: Use it Wisely! How many molecules of CO 2 are in g of CO 2? g CO 2 x 1 mol CO g CO 2 x 6.02 x molecules 1 mol CO 2 CO 2 = 1.69 x molecules CO 2 How many O atoms are in g of CO 2? g CO 2 x 1 mol CO 2 x 6.02 x mcs CO 2 x 2 O atoms g CO 2 1 mol CO 2 1 mc CO 2 = 3.37 x O atoms compounds convert directly to molecules elements convert directly to atoms compounds to atoms take an extra step!

33 Avogadro s Number: Use it Wisely! 33 How many total atoms are in 4.5 g of H 2 SO 4? 4.5 g H 2 SO 4 x 1 mol H 2 SO g H 2 SO 4 x 6.02 x mcs H 2 SO 4 1 mol H 2 SO 4 x 7 atoms 1 mc H 2 SO 4 = 1.93 x atoms How many H atoms are in 4.5 g of H 2 SO 4? 4.5 g H 2 SO 4 x 1 mol H 2 SO g H 2 SO 4 x 6.02 x mcs H 2 SO 4 1 mol H 2 SO 4 x 2 H atoms 1 mc H 2 SO 4 = 5.53 x H atoms A question using this concept will be on the exam!!!

34 Practice Problems: Molar Mass Find the molar mass of the following compounds: a. C 3 H 7 OH b. cobalt(iii) bromide c. silicon dioxide d. C 8 H 9 NO 2 (active ingredient in Tylenol) 2. Find the number of moles of each substance: a g iodine b g beryllium nitrate c g C 9 H 20 d. 659 g C 2 H 5 OH

35 Practice Problems: Avogadro s # 35 Find the number of grams in the following compounds: a x molecules of C 6 H 14 b x atoms of aluminum c moles of ammonium hydroxide

36 Practice Problems: Mass Relationship 1. Potassium sulfate is found in some fertilizers as a source of potassium. How many grams of potassium can be obtained from g of the compound? 36 Practice Problems: Percent Composition 1. Calculate the percent composition of each compound: a. octane (C 8 H 18 ) b. aluminum acetate c. calcium dihydrogen phosphate d. chromium(ii) chloride

37 Practice Problems: Empirical Formula A compound is found to contain 63 % manganese and 37 % oxygen. Find the empirical formula of the compound. 2. A compound contains g of nitrogen and g of oxygen. Find its empirical formula. 3. A compound has % carbon, 4.95 % hydrogen and % oxygen and has a molar mass of 366 g/ mol. What is the empirical formula for this compound? What is the molecular formula?

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