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Atomic Mass l Atoms are so small, it is difficult to discuss how much they weigh in grams. l Use atomic mass units. l an atomic mass unit (amu) is one.

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Presentation on theme: "Atomic Mass l Atoms are so small, it is difficult to discuss how much they weigh in grams. l Use atomic mass units. l an atomic mass unit (amu) is one."— Presentation transcript:

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2 Atomic Mass l Atoms are so small, it is difficult to discuss how much they weigh in grams. l Use atomic mass units. l an atomic mass unit (amu) is one twelth the mass of a carbon-12 atom. l The decimal numbers on the table are atomic masses in amu.

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4 They are not whole numbers l Because they are based on averages of atoms and of isotopes.

5 mass Cu atom 6909 Cu x62.93 = u 3091 Cu x64.93 = u Total mass= u Average mass= total mass/10000=63.55 u

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7 The Mole l The mole is a number. l A very large number, but still, just a number. l x of anything is a mole l A large dozen. l The number of atoms in exactly 12 grams of carbon-12.

8 The Mole l 1 mol of atoms of any element weighs the number value of atomic weight in g. l 1 mol Cu weighs g l 1 mol Ga weighs g

9 Molar mass, M l Mass of 1 mole of a substance. l Often called molecular weight. l To determine the molar mass of an element, look on the table. l To determine the molar mass of a compound, add up the molar masses of the elements that make it up.

10 Find the molar mass of CH 4 : CH4 → C + 4 H m CH4 = m C + m H M CH4 = M C + 4×M H Mg 3 P 2 : M = 3 × M Mg + 2 × M P l Ca(NO 3 ) 3 l Al 2 (Cr 2 O 7 ) 3 l CaSO 4 · 2H 2 O

11 Number of moles, n Avogadro’s number N av =6.02x10 23 mol -1

12 Number of moles, number of atoms in 10 g Al? Number of moles, mass of 5.00x10 20 atom Co?

13 Number of molecules of isopentyl acetate in 1 ug? Number of carbon atoms?

14 Mass of CO 3 2- in 4.86 mol CaCO 3 ?

15 Percent Composition l Percent of each element a compound is composed of. l What is the percent composition of C 2 H 5 OH?

16 Suppose we have 1 mol C 2 H 5 OH Sum of all percentages=100% 100 g of C 2 H 5 OH contains g C, g H and g O.

17 Find the percent compositions of the following compounds: l Al 2 (Cr 2 O 7 ) 3 l CaSO 4 · 2H 2 O

18 l From percent composition, you can determine the empirical formula. l Empirical Formula the lowest ratio of atoms in a molecule. A sample is 59.53% C, 5.38%H, 10.68%N, and 24.40%O, what is its empirical formula? CHNO sample mass=100 g59.53 g5.38 g10.68 g24.4 g n=m/M59.53/ / / /16.00 n Divide by smallest value E.F. C 13 H 14 N 2 O 4

19 Empirical Formula and Molecular Formula l H 2 O 2 EF: HO l Such compound doesn’t exist! l MF = n × EF H 2 O 2 = 2 (HO) l M MF =n × M EF 34 = 2 × 17 l 2×M H + 2×M O = 2 (M H +M O ) l Alkenes: C 2 H 4, C 3 H 8, C 4 H 8, C 5 H 10 … l EF : CH 2

20 ClCH sample mass=100 g71.65 g24.27 g4.07 g n=m/M71.65/ / /1.008 n Divide by smallest value112 E.F. CH 2 Cl M.F. C 2 H 4 Cl 2

21 Elemental Analysis A substance is known to be composed of C, H and nitrogen. When g of this substance is reacted with oxygen, g CO 2 and g H 2 O. Find the empirical formula.

22 l Assumptions: 1) All C in sample converted into CO 2.

23 2) All H in sample converted into H 2 O.

24 CHN mass g g g n=m/M / / /14.00 n Divide by smallest value151 E.F. CH 5 N

25 Chemical Equations l Are sentences that describe what happens in a chemical reaction. Reactants  Products CH 4 + O 2  CO 2 + H 2 O l Redistribution of bonds, atoms don’t change! l Equations should be balanced: Have the same number of each kind of atoms on both sides. l Law of conservation of mass. CH 4 + 2O 2  CO H 2 O Balancing by inspection.

26 Abbreviations ( s )  ( g )   l  l ( aq ) l heat  l catalyst

27 Practice Ca(OH) 2 + H 3 PO 4  H 2 O + Ca 3 (PO 4 ) 2 Cr + S 8  Cr 2 S 3 KClO 3 ( s )  Cl 2 ( g ) + O 2 ( g ) l Solid iron(III) sulfide reacts with gaseous hydrogen chloride to form solid iron(III) chloride and hydrogen sulfide gas. Fe 2 O 3 ( s ) + Al( s )  Fe( s ) + Al 2 O 3 ( s )

28 Stoichiometric Calculations What mass of oxygen will react completely with 96.1 g of propane? C 3 H 8 + O 2  CO 2 + H 2 O l Balance the equation! C 3 H O 2  CO H 2 O l Language of moles, not grams!

29 C 3 H O 2  CO H 2 O ?

30 Grams of CO 2 produced? C 3 H O 2  CO H 2 O ?

31 Mass of CO 2 absorbed by 1.00 kg LiOH (s) ? ?

32 Antiacids

33 ? ?

34 Examples l One way of producing O 2 ( g ) involves the decomposition of potassium chlorate into potassium chloride and oxygen gas. A 25.5 g sample of Potassium chlorate is decomposed. How many moles of O 2 (g) are produced? l How many grams of potassium chloride? l How many grams of oxygen?

35 Examples l A piece of aluminum foil 5.11 in x 3.23 in x in is dissolved in excess HCl(aq). How many grams of H 2 ( g ) are produced? How many grams of each reactant are needed to produce 15 grams of iron form the following reaction? Fe 2 O 3 ( s ) + Al( s )  Fe( s ) + Al 2 O 3 ( s )

36 Examples K 2 PtCl 4 ( aq ) + NH 3 ( aq )  Pt(NH 3 ) 2 Cl 2 ( s )+ KCl( aq ) l what mass of Pt(NH 3 ) 2 Cl 2 can be produced from 65 g of K 2 PtCl 4 ? l How much KCl will be produced? l How much from 65 grams of NH 3 ?

37 Limiting Reactant

38 1 2 5 ?=10 mol NH ?=6 mol NH 3 15 (not available) H 2 is LR.

39 3 mol nitrogen 12 mol hydrogen ?=6 mol NH ?=8 mol NH (not available) N 2 is LR.

40 Limiting Reactant l Reactant that determines the amount of product formed. l The one you run out of first. l Makes the least product. N 2 + H 2  NH 3  What mass of ammonia can be produced from a mixture of 100. g N 2 and 500. g H 2 ?  How much unreacted material remains?

41 ?=7.14 mol NH ?=165.3 mol NH 3

42 Reaction Yield l The theoretical yield is the amount you would make if everything went perfect. l The actual yield is what you make in the lab. Percent Yield

43 68.5 kg CO is reacted with 8.60 kg H 2. Calculate the percent yield if the actual yield was 3.57x10 4 g CH 3 OH.

44 ?= mol CH 3 OH ?= mol CH 3 OH

45 Examples l Aluminum burns in bromine producing aluminum bromide. In a laboratory 6.0 g of aluminum reacts with excess bromine g of aluminum bromide are produced. What are the three types of yield. 2 Al + 3 Br 2  2 AlBr 3

46 Examples Years of experience have proven that the percent yield for the following reaction is 74.3% Hg + Br 2  HgBr 2 If 10.0 g of Hg and 9.00 g of Br 2 are reacted, how much HgBr 2 will be produced?

47 Examples l Commercial brass is an alloy of Cu and Zn. It reacts with HCl by the following reaction Zn(s) + 2HCl(aq)  ZnCl 2 (aq) + H 2 (g) Cu does not react. When g of brass is reacted with excess HCl, g of ZnCl 2 are eventually isolated. What is the composition of the brass (i.e. Zn% and Cu% )?


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