Useful only for measuring the mass of very small objects atoms and molecules!

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1 Chapter 9 Chemical Composition (Moles) Which weighs more, 1 atom of He or 1 atom of O? Units of mass: Pound Kilogram Atomic mass unit (AMU) There are others! 1 amu = 1.66 x grams = mass of a proton or neutron (recall, they both weigh about the same. Useful only for measuring the mass of very small objects atoms and molecules!

2 1 atom of C-12 weighs 12 amu s There are 3 isotopes of Carbon: C-12 C-13 C-14 The weighted average of all isotopes of Carbon is amu / atom. Which isotope is more abundant? How many amu s does 1 atom of Li weigh? Fe? Problem: How many amu s does 150 atoms of C weigh? Problem: How many amu s does 2.2 x atoms of Mg weigh? Problem: How many atoms of Li are in a pile that weigh 3.00 x amu?

3 How many amu s would 1 molecule of H 2 O weigh? Problem: How many amu s would 25 molecules of H 2 O weigh? Amu s are a very small unit of measure, and we don t use them very often.

4 We can measure matter either by counting the particles, or weighing the substance. Mole (Mol) the unit in chemistry used to measure a quantity of matter. 1 mole = 6.02 x particles (atoms or molecules) called Avogadro s Number 1 mole = atomic mass (grams)

5 1 mol of sulfur, mercury, iodine and iron.

6 Problem: How many grams would 1 mole C weigh? 1 mole He? 1 mole U? Which element would have the most number of atoms? How big is 6.02 x 10 23? Would 1 mole of rice fill up the class? The gym?

7 Problem: Write 2 conversion factors for Carbon Problem: Write 2 conversion factors for Magnesium Problem: You own 15 grams Mg How many moles of Mg do you have? How many atoms of Mg do you have? Problem: Write 2 conversion factors for Fe You own 3.8 x atoms Fe How many moles of Fe do you own? How many grams of Fe do you own?

8 1 mole of methane, CH 4 = 6.02 x molecules of methane. What would 1 mole CH 4 weigh?

9 Molar mass = molecular mass = molar weight = molecular weight What is the difference between atomic mass and molar mass? 5 conversion factors for water: 1 mole water = 6.02 x molecules of water 1 mole water = 18.0 grams 1 molecule water = 3 atoms 1 molecule water = 2 atoms H 1 molecule water = 1 atom O Problem: You have 36 grams H 2 O. How many moles of water are in the cup? How many molecules of water are in the cup? How many atoms are in the cup? How many atoms of H are in the cup?

10 Problem: You own 5.4 x molecules of sugar (C 12 ) determine 6 conversion factors for sugar. Possible Conversions: 1) How many moles of sugar? 1 mole C 12 = 6.02 x molecules C 12 2) How many grams of sugar? 1 mole C 12 = g C 12 1 mole C 12 = 12 mole C 1 mole C 12 = 22 mole H 1 mole C 12 = 11 mole O 3) How many atoms? 4) How many atoms of O? 1 molecule C 12 = 12 atoms C 1 molecule C 12 = 22 atoms H 1 molecule C 12 = 11 atoms O 1 molecule C 12 = 45 atoms

11 1) How many moles of sugar? 2) How many grams of sugar? 3) How many atoms? 4) How many atoms of O? 1)?moles sugar = 5.4 x molecules x 1 mole = 9.0 x 10 4 mole sugar 6.02 x molecules 2)? g sugar = 5.4 x molecules x 1 mole x 342 g = 3.1 x 10 7 g sugar 6.02 x molecules 1 mole 3)? atoms = 5.4 x molecules x 45 atoms = 2.4 x atoms 1 molecule 4)? atoms O = 5.4 x molecules x 11 atoms O = 5.9 x atom O 1 molecule

12 Percent composition of compounds % by mass of an element in a compound = g element in 1 mole of compound x 100 g in 1 mole of compound What percent by mass is Carbon in ethanol, C 2 H 6 O? Determine the mass percent of N in ammonium phosphate fertilizer. Determine the mass percent of N in ammonia fertilizer. Which is the better fertilizer? Determine the mass percent of ammonium in ammonium phosphate.

13 Empirical Formula the lowest, whole number ratio of atoms in a molecule. Is the formula of glucose, C 6 H 12 O 6 empirical? What does C 6 H 12 O 6 reduce to? H 2 O? N 2 O 5? H 2 O 2? If you were told that the empirical formula of glucose is CH 2 O is the empirical formula of glucose, what would you need to do to determine the actual formula of the molecule?

14 What does H 2 O mean? 2 atoms H : 1 atom O or, as a ratio: 20 atoms H : 10 atoms O 24 atoms H : 12 atoms O 2 dozen atoms H : 1 dozen atoms O 2 mole H : 1 mole O Does it also indicate mass ratio? 2 grams H : 1 gram O???

15 5.2 grams of charcoal (carbon) is burned, and 19.0 grams of product is formed. What is the empirical formula of the product?

16 A substance contains 4.2 g Al and 3.8 g O. Determine its empirical formula.

17 The percent of O in H 2 O is 89%, the percent of H is 11%. If you had 100 grams of water, what number of grams of O do you have? H? If you had 200 grams of water, what number of grams of O do you have? H? What is the formula of water if you have 100 grams? 200 grams? Sometimes, the percent by mass, rather than grams is given: Problem: A sample of insecticide contains: 19.7 % C 22.9 % N 5.0 % H 52.4 % O Determine the empirical formula of this insecticide

18 Nylon is composed of: 63.7 % C 12.4 % N 9.8 % H 14.1% O Determine the empirical formula of Nylon

19 Molecular Formula the actual formula of a molecule. A substance has an empirical formula of HO, and has a molar mass of 34.0 g/mole. What is its molecular formula? HO weighs 17 g/mole H 2 O 2 weighs 34 g/mole H 3 O 3 weighs 51 g/mole H 4 O 4 weighs 68 g/mole.....

20 Problem: a substance is composed of: 3.0 g carbon 0.5 g H 4.0 g O and has a molecular mass of 180 g/mole. Determine the empirical and molecular formula of this compound.

21 A compound containing phosphorus and oxygen has a molecular mass of 284 g/mole. It is found to contain (by mass): 43.7 % Phosphorus 56.3% Oxygen Determine the empirical and molecular formula of this compound.

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