Chemistry/Hart. Moles

Transcription

1 Moles How can we count how many atoms or molecules are in a piece of matter if we can t see them? How can we count how many atoms or molecules are in a piece of matter if they have different masses? What can we measure in the laboratory that will help us? What is the common currency? MOLES! 1 mole = the amount of pure substance that contains as many particles (atoms, molecules, or fundamental units) as there are atoms in exactly 12 grams of carbon-12 (agreed upon by chemists and physicists in 1960/61) = 6.02 x particles = Avogadro s number Mole comes from molekül (German) = molecule Molecule comes from molecula (New Latin), meaning very small specimen Background: Lorenzo Romano Amedeo Carlo Avogadro ( ) - Italian physics professor Avogadro's hypothesis - now a law "Equal volumes of gases under the same conditions have equal numbers of molecules." universal container Late 1800s chemists developed scale of relative atomic masses of gases, based on 1/16 of the average atomic mass of oxygen 1920s physicists developed relative atomic masses based on 1/16 of the oxygen-16 atom chemists and physicists - agreed to switch to carbon-12 as the standard, setting its atomic mass at 12 (pragmatic reasons carbon-12 was the standard in mass spectroscopy, and close to chemistry s oxygen standards) - 1 -

2 number of particles moles Moles in conversion factors 6.02 x particles 1 mole mass (g) moles average atomic mass in g 1 mole number of particles mass (g) both of the above conversion factors number of atoms in a compound # atoms 1 compound Math and the Mole (# particles/mole) 1. How many fingers are there on 1 person? 2. How many fingers are there on 1 dozen people? 3. How many fingers are there on 3 dozen people? 4. How many fingers are there on 1 mole of people? - 2 -

3 5. How many fingers are there on 3.12 moles of people? 6. How many F atoms in 3.12 moles of F? 7. How many moles of F do you have if you have 2.45 x atoms of F? (mass/mole) Hint: if problem includes numbers of atoms or other representative particles and moles, use Avogadro s number: 6.02 x particles If problem includes grams and moles use the periodic table to find molar mass. 8. How many grams of F are in 3.89 moles of F? 9. How many moles of F atoms are in 45.6 g of F? 10. How many F atoms are in 65.8 g F? (# particles/mass) 11. What is the mass, in grams, of 7.62 x F atoms? - 3 -

4 (# atoms/compound) 12. How many F atoms are in 3.84 moles of MoF 6 molecules? Molar Mass Molar mass = mass, in grams, of 1 mole of a substance (6.02 x particles) expressed in g/mol Molar mass is numerically equal to average atomic mass in amus (atomic mass units). So, molar mass of oxygen (O) = mass of 1 mole of O atoms = mass of 6.02 x atoms = g/mol molar mass of lead (Pb) = mass of 1 mole of Pb atoms = mass of 6.02 x atoms = g/mol How do we determine the molar mass of compounds? Add up the molar masses of all elements in the compound, taking into account the number of moles of each element. e.g. What is the molar mass of Na 3 PO 4? 3 moles of g/mol = 3 mol x g mol 1 mole of g/mol = 1 mol x g mol 4 moles of g/mol = 4 mol x g mol Add these together = g = molar mass of Na 3 PO 4 How do we use molar mass of a compound in a conversion problem? The same way we use molar mass of an atom - 4 -

5 (#atoms/compound and molar mass of compounds) 13. How many Na atoms are in 252 g of Na 3 PO 4? Moles Warmups 1. How many moles of Cu are in 4.95 g of Cu? A: mol Cu 2. How many atoms of Cu are in 5.00 mol of Cu? A: 3.01 x Cu atoms 3. How many grams of Na are in 5.29 moles Na? A: 122 g Na 4. How many moles of Ba are in 5.25 x atoms of Ba? A: 8.72 x mol Ba 5. What is the mass (in g) of 7.5 x atoms of Ni? (hint: 2 conversion factors) A: 7.3 x 10-7 g Ni 6. How many atoms of Na are in 25 g of Na? (hint: 2 conversion factors) A: 6.5 x Na atoms - 5 -

6 7. What is the molar mass of calcium chloride (CaCl 2 )? A: g/mol 8. How many O atoms are in 2.9 moles of SO 3? A: 5.2 x O atoms 9. How many Ca atoms are in 532 g of Ca? A: 7.99 x Ca atoms 10. How many grams of Pb are in 4.20 x atoms of Pb? A: 14.5 g Pb 11. How many K atoms are in 16.2 g of K 2 S? A: 1.77 x K atoms 12. What is the molar mass of barium nitrate (Ba(NO 3 ) 2? A: g/mol 13. What is the mass of 5.26 mol of NaOH? A: 210. g NaOH 14. What is the mass of 5.62 x formula units of CuSO 4? A: 1.49 x 10-2 g - 6 -

7 15. How many hydrogen atoms are in 562 g of ethanol (C 2 H 6 O)? A: 4.41 x H atoms Molar Calculations relating to Chemical Formulas Warmup: What is the molar mass of Pb 3 (PO 4 ) 2? 1. Determine percent composition (by mass) of all elements present in a compound: Find the molar masses of each element in the compound. Find the total molar mass. Calculate: % composition = molar mass of element x 100% total molar mass of compound e.g. What is the % composition (by mass) of NaCl? CaCl 2? Apply % composition: e.g. What mass of Na is present in g NaCl? - 7 -

8 Mass Percent Warmups 1. Copper (I) sulfide is found in nature as the mineral chalcocite, a copper ore. What is the mass percent of copper in chalcocite? A: 79.85% Cu 2. Chalcopyrite has the formula CuFeS 2. What is the mass percent of copper in chalcopyrite? A: 34.6% Cu 3. Will you get more copper from the same mass of pure chalcopyrite or pure chalcocite? Explain your answer. 2. Convert % composition (mass percent) empirical formula a. Assume a 100 g sample use same numbers as grams rather than %. b. Perform mass mole conversions. c. Divide each result (mole) by the smallest result present ( mole ratio). d. Look for whole number ratio. Rhyme to remember order of steps to convert % composition empirical formula: Percent to mass Mass to mole Divide by small Times till whole - 8 -

9 Practice: 1. One of the components of fresh alkaline batteries is a black powdery compound, made of 63% manganese and 37% oxygen. What is the compound s empirical formula? 2. While analyzing a dead alkaline battery, Antonio found a compound that is made of 70.0% manganese and 30.0% oxygen. What is its empirical formula? Empirical Formula Warmups 1. During a winter vacation, you work at a ski resort covering icy sidewalks with a substance containing 26.2% N, 7.5% H, and 66.3% Cl. What is the formula for this compound? A: NH 4 Cl - 9 -

10 2. Phosphorus forms two oxides. One has 56.34% P and 43.66% O. The other has 43.64% P and 56.36% O. What are the empirical formulas for these compounds? A: P 2 O 3 and P 2 O 5 3. What is the empirical formula for a compound that contains 26.56% potassium, 35.41% chromium, and 38.03% oxygen? A: K 2 CrO 7 4. What is the empirical formula for a compound that contains g of iron and g of sulfur? A: FeS

11 3. How to determine molecular formula Compound Empirical vs. Molecular Formula Empirical formula Empirical molar mass Molecular molar mass Molecular formula Formaldehyde CH 2 O g g CH 2 O Acetic acid CH 2 O g g C 2 H 4 O 2 Glucose CH 2 O g g C 6 H 12 O 6 To find the molecular formula of glucose, divide the molecular molar mass by the empirical molar mass, round to the nearest whole number. (molecular) molar mass = g = ~ 6 empirical molar mass g then multiply subscripts by 6 => C 6 H 12 O 6 to get the molecular formula Practice Problem Determining molecular formula Hydrazine is 87.42% N and 12.58% H. The (molecular) molar mass of hydrazine is 32.0 g/mol. a) What is its empirical formula? b) What is its molecular formula?

12 Solution Concentration Percent by Mass Mass % of component = mass of component in solution x 100% total mass of solution e.g. In order to maintain a sodium chloride (NaCl) concentration similar to ocean water, an aquarium must contain 3.6 g NaCl per g of water. What is the percent by mass of NaCl in the solution? 3.6 g NaCl x 100% = 3.6 g NaCl x 100% g H 2 O g NaCl g total = 3.5% e.g. A solution contains 2.7 g of CuSO 4 in 75 ml of solution. Assume the density of the solution is 1.0 g/ml. What is the mass percent of the solution? 2.7 g (1 ml) x 100% = 3.6% 75 ml 1.0 g Molarity Molarity = the # moles solute dissolved in 1 L of solution Molarity: the concentration of solution in moles solute L solution not solvent See Moles WS 6 for notes and practice problems related to molarity