9/18/2013. Scientists represent atoms by using different colored circles, called a model.

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1 Pre-Lab Notes Lab Title: Behavior of Gases: Molar Mass of a Vapor Purpose: To determine the molar mass of a gas from a knowledge of its mass, temperature, pressure, and volume. Each element is unique. Elements have their own physical and chemical characteristics. Below is an example of each element in model format. An element in model form is a single, colored circle. Scientists represent atoms by using different colored circles, called a model. Elements are organized in a table called the periodic table of the elements. Different element have a different number of protons and thus different physical & chemical characteristics. The atomic number indicates the number of protons. Most substances are chemical combinations of elements (atoms), called compounds. Below are some models of different compounds. Compounds are more than one colored circle connected by a stick known as a bond. 1

2 Diatomic Elements Some elements are naturally compounds! Poly (many) atomic (atoms) Polyatomic elements consist of several like atoms bonded together Memorize these: 7 th Heaven Rule 7 Diatomic elements: H 2, N 2, O 2, F 2, Cl 2 Br 2, I 2 Review Element or Compound? Polyatomic Sulfur & Sulfur Polymer A)Element B)Compound C)Mixture 2

3 A)Element B)Compound C)Mixture A)Element B)Compound C)Mixture A)Element B)Compound C)Mixture Calculating AMU As you learned last year, there are different types of atoms known as elements. Each element has a different mass and it is called the amu = atomic mass units Another name for amu is molar mass Molar mass is obtained by summing the masses of the component atoms. 3

4 On Your Periodic Table Color Code Those 7 Elements Remember 7th Heaven Rule For example: NH 3 has the following amu: N = 1 atom x amu = amu H = 3 atoms x 1.01 amu = 3.03 amu total amu = amu or molar mass Ch (Brown & LeMay) To find the percent of anything. First find the total part wanted. Next count the total number, the whole. Then divide the part over the whole and multiply by 100 % Percent composition = Part x 100% whole This can be used for calculating different types of percentage compositions. What is the % N to % H of Windex, which is ammonia, (NH 3 )? Notice no numbers are given, therefore use the compound s amu. part x 100% whole N = 1 atom x amu = x 100% = % N H = 3 atoms x 1.01 amu = 3.03 x 100% = % H 4

5 The percentages may not always total to 100% due to rounding, for example if you go to 1 decimal spot 82.2% N % H 99.9 % total But if you go to 2 decimal spots you get closer to 100% 82.21% N % H % total Overall your numbers should add up close to 100% Calculating % Composition By Given Masses According to the Law of Constant Composition, any sample of a pure compound always consists of the same elements combined in the same proportion by mass. Percent composition can be determined of each element in a compound by its mass: % Composition = part mass x 100% whole mass This formula can be applied by using the formula of the compound or by experimental mass analysis of the compound A sample of butane (C 4 H 10 )--lighter fluid-- contains 288 g carbon and 60 g hydrogen. Find %C and %H in butane 5

6 Percent composition = Part x 100% whole 288 g C + 60 g O = 348 g Part Part whole The Mole 288 g C (C part) x 100 % = 82.8 % C 348 g (whole) 60 g O (O part) x 100% = 17.2 % O 348 g (whole) % Molar Mass The mass of one mole of atoms of any element is the molar mass which is numerically equal to the atomic mass unit (amu), but in grams Molar mass = g = 1 amu 1 mole Therefore CO 2 has an amu = or g 1 mol Your and your lab partner will be assigned either an element or a compound. You will need to place 1 mole of the element or compound into a container. Make sure you do the following: label the bottle with the correct formula Label the bottle 1 mole You have a permanent marker in your lab drawer. Now you & your lab partner draw molecular level pictures of the mole containers. 6

7 Using The Mole Map Converting from grams to moles EXAMPLE: A student weighs out 88 grams of solid CO 2 (dry ice), how many moles does the student have? First, find the amu of CO 2 : C = 1 atom x amu = amu O = 2 atoms x amu = amu CO 2 total amu = amu CO 2 = 44g 1 mol Then convert from grams to moles Use your mole map 88 grams CO 2 Using The Mole Map 7

8 (What s Given) x 1 mole (molar mass) Converting To Particles 88 g CO 2 x 1 mole = 2 moles CO 2 44 g Next, convert from mol to molecules: A particle can be defined as: an atom, a molecule, or formula unit (ionic compounds) Use your mole map 2 mole CO 2 (moles calculated) x (Avogadro s #) 1 mole 2 mole CO 2 x 6.02 x molecules 1 mole = x molecules CO 2 8

9 Chalk is calcium carbonate (CaCO 3 ). I want you to calculate the number of moles and the number of particles of CaCO 3 it takes to write your FULL name (First, Middle, & Last) on a piece of paper. Avogadro s Law Ch 10.3 In 1811 Italian chemist Avogadro postulated equal volumes of gases at the same temperature and pressure contains the same number of particles V = k n n = moles, k = proportionality constant At constant temp and press, volume is proportional to moles Again: PV = nrt Avogadro s Law One mole (6.02x10 23 particles) of ANY ideal gas at standard temperature and pressure, (STP = 0 o C, 1 atm) occupies a volume of 22.4 L 22.4 L is about the volume of a basketball. Avogadro s Law You and your partner calculate the volume (L) at STP (0 degrees C and 1 atm pressure) for 1 mole. PV = nrt Or V = nrt P Note: You have use the units Kelvin Note: R constants on your AP Cheat Sheet Which one to use? Hint: look at units V = nrt = P (1.000mol)( L atm/k mol)(273.2 K) atm V = L Does this look familiar??? This volume is the molar volume of an ideal gas (0 C and 1 atm) The conditions 0 C and 1 atm, are called standard temperature and pressure (abbreviated STP) 9

10 It Does NOT Matter The Type of Gas (at STP) AT STP: To go from moles L or L moles we will be using the top section of the mole map. A reaction produces 893 Liters of air, how many moles at STP? At STP how many Liters in 3.45 moles of O 2? The Ideal Gas Equation 10.4 The Ideal Gas Law The ideal gas law: PV = nrt Where R is the combined proportionality constant is called the Universal Gas Constant: R = L atm K mol A gas that obeys this equation is said to behave ideally. The ideal gas equation is best regarded as a limiting law it expresses behavior that real gases approach at low pressures and high temperatures (an ideal gas is a hypothetical substance). Most gases obey the ideal gas equation closely enough at pressures below 1 atm. The Universal Gas Constant can be calculated by using various units, therefore it is important that you note the units in the question being asked and therefore the correct gas constant. R = L atm K mol R = 62,400 ml mm Hg K mol Some conversions to consider: L ml, atm mmhg, degrees C K 10

11 Conversions: 1000 ml = 1 L, 1 atm = 760 mmhg, you either add 273 or subtract 273 for Celsius/Kelvin. A gas sample of 0.896g occupies a volume of 524 ml at 730 mm Hg and 28 degrees Celsius. What is the number of moles moles? What is the Molar Mass (MM)? NOTE: Which constant to use? R = L atm K mol R = 62,400 ml mm Hg K mol You can use either one, but the second one involves less conversions (note mol = n) R = 62,400 ml mm Hg K mol V = 524 ml, P =730. mm Hg,T =28 degrees Celsius PV = nrt, thus PV =n AND = 301 K RT (730. mm Hg)(524 ml) = n (62,400 ml mm Hg/ K mol)(301 K) mol = n MM = g/mol One can identify an unknown volatile gas by using the Universal Gas Law and the mass of the unknown. A gas sample of 0.896g occupies a volume of 524 ml at 730. mm Hg and 28 degrees Celsius. What is the number of moles moles? What is the Molar Mass (MM)? mol = n and g of the sample: g / mol = 43.9 g/mol PV = nrt and Density = mass/volume A short cut, Moles put dirt over their pee MM = DRT = mass RT P V P Ch 3.6 Mole-Mole Relationships Stoichiometry is stoicheion meaning elements and metron meaning to measure... 11

12 Mole-Mole Relationships In order to calculate an amount of reactant or product at the end of a reaction you will need to do the following: 1. Balance the reaction. 2. Use the mole ratio from the balanced equation to compare the moles known to the moles unknown. 3. Multiple/divide. Using Mole Ratios Phosphorus is placed in a flask of chlorine gas, heat and light is given off forming phosphorus trichloride: If 0.06 moles of Cl 2 gas how many moles of PCl 3 produced? P Cl 2 --> 4 PCl 3 Use Your Stoichiometry Map If 0.06 mol of Cl 2 gas how many mol of PCl 3 produced? P Cl 2 --> 4 PCl mols? mols? If 0.06 mol of Cl 2 gas how many mol of PCl 3 produced? P Cl 2 --> 4 PCl mols? mols? 12

13 Now You Try Ammonia (NH 3 ) is used in huge quantities as a fertilizer. It is manufactured by combining nitrogen and hydrogen according to the following equation: N H 2 2 NH mol? mol 1.30 mol H 2 x 2 mol NH 3 = mol NH 3 3 mol H 2 N H 2 2 NH 3 How many moles of NH 3 can be made from 1.30 mol H 2? Mass-Mass Calculations Steps for Calculating the Masses of Reactants & Products in Chemical Reactions STEP 1 Balance the equation for the reaction. STEP 2 Convert the masses of reactants or products to moles STEP 3 Use the balanced equation to set up the appropriate mole ratio(s). STEP 4 Use the mole ratio(s) to calculate the number of moles of the desired reactant or product. STEP 5 Convert from moles back to mass. How many grams of Cl 2 will react with 1.24 g of P 4? P Cl 2 4 PCl g? g 13

14 P Cl 2 4 PCl g? g g P 4 mol P 4 mol: mol ratio g Cl g P 4 x 1 mol P 4 = mol P g mol P 4 x 6 mol Cl 2 = mol Cl 2 1 mol P mol Cl 2 x g Cl 2 = 4.26 g Cl 2 1 mol Cl 2 14