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The Mole Chapter 7 Chemical Quantities What’s a mole anyway? Well do you remember how to find the molecular mass of a substance? Get out a periodic table. Find the atomic mass of carbon. What is it? 12 What unit goes at the end? Amu (u) or atomic mass units. Well it so happens that if you take that same number and put grams instead of u, you have one mole of that substance. In other words, trade u for g and you now have a mole of carbon. So one mole of carbon is 12 grams of carbon. Scientists use moles because atoms and molecules are too tiny to count. Instead electronic balances are used to measure atoms and molecules in grams. How many grams of carbon in 4 moles of carbon? 48 g How many moles of carbon are there in 500 g of carbon? 500 g (1 mol) ( 12 g) How many moles of carbon are there in 500 g of carbon? 41.6 mole How many moles of oxygen are there in 500 g of oxygen? 500 g (1 mol) ( 16 g) How many moles of oxygen are there in 500 g of oxygen? 500 g (1 mol)= 31.25 mol ( 16 g) How many grams of calcium must be weighed out in order to obtain a sample containing 3 moles of atoms? a) 40 g b) 3 x 6 x 1023 g c) 120 g 120 g What is the difference between molecular mass and molar mass? Molecular mass is in atomic mass units; molar mass is in grams. Find the number of moles in 24 g of CO 24 g (1 mole) ( 28 g ) = 0.86 moles of CO Convert 872 g of ammonium oxide into moles. First find the correct formula for ammonium oxide. (NH4)2O Find the molar mass of ammonium oxide (NH4)2O N – 2 x 14 = 28 H–8x1= 8 O – 1 x 16 = 16 Find the molar mass of ammonium oxide (NH4)2O = 52 grams Set up a conversion factor to change from 872 g to moles. Conversion factor 872 g ( 1 mol) 52 g = 16.8 What is Avogadro’s number? 6.02 x10 23 units of a substance 44. Which of the following equals 9.2 x 1024 divided by 6.3 x 104? A. 1.46 X 1024 B. 1.46 X 10 20 C. 1.46 X 10 6 D. 1.46 X 1028 Avogadros's number, 6.02 x 10 23 , is also called a mole, in the same way that 500 sheets of paper is called a ream, or 144 is referred to as a gross. http://www.wiredchemist.com/chemis try/instructional/chem_modules_mole _lecture.html How many ping-pong balls are there in a mole of ping-pong balls? What would their molar mass be if one ping-pong ball weighs 1.0 g? 6.02 x 1023 ping-pong balls in a mole of ping- pong balls. If each ball weighs 1.0 grams, the molar mass will be 6.02 x 1023 g. We would normally express this as 6.02 x 1023 g/mole. Are you saying that we can have a mole of anything? Sure, can't you have a dozen pencils, a dozen ducks, a dozen bricks? However, we normally use the term mole in dealing with atomic particles--atoms, ions, molecules. How many atoms are present in 31 g of phosphorus? a) 1 mole b) 6.022 x 1023 c) Avogardro's number d) a, b, and c All three are correct Your Zork particle counter has just informed you that there are 3 x 1020 atoms of radon in your basement. How many moles of radon is this? 3 x 1020 particles/ 6 x 1023 particles per mole) = 5 x 10-4 mole. Your lab partner is staring out the window as usual when he suddenly exclaims, "Gadzooks! There are 1 x 10-23 moles of dogs running down the street!" Could he be right for a change? Yes, 1 x 10-23 moles is: 1 x 10-23 moles x 6 x 1023 dogs per mole = 6 dogs. If you like to solve problems with dimensional analysis you would probably set this up as: 1 x 10-23 moles x 6 x 1023 dogs / 1 mole = 6 dogs. What about volume? The mole road map shows a conversion for volume. volume 1 mole of any gas at STP = 22.4 L What’s STP? STP means standard temperature and pressure. What’s STP? Standard temperature is 0° C Standard pressure is 1 atm or 101.3 KPa Find the volume that 24 g of CO occupies at STP. 24 g (1 mole) ( 28 g ) = 0.86 moles of CO Find the volume that 24 g of CO occupies at STP. 0.86 moles (22.4 L) ( 1 mol ) = 19.264 Liters Find the number of atoms in 24 g of CO 0.86 mol (6.02 x 1023) (1 mol) 5.2 x 1023 molecules of CO Find the density of one mole of CO gas at STP. Density = Mass divided by volume D = 28 g = 22.4 L Find the density of one mole of CO gas at STP. Density = Mass divided by volume D = 28 g = 1. 25 g/L 22.4 L Determine the percent composition of Fe(OH)2 Fe – 1 x 55.8 = 55.8 O – 2 x 16 = 32 H–2x1= 2 Molar mass = 89.8 Divide element mass by molar mass, then multiply by 100. Fe – 55.8 x 100 89.8 O – 32 x 100 89.8 Divide element mass by molar mass, then multiply by 100. H –2 x 100 89.8 Percent compositions % composition Fe = 62.1% % composition 0 = 35.6% % composition H = 2.2% Add them up. Do they total 100? Empirical formulas Which of the following are empirical formulas? HCl K2C2O4 Pb2O4 Empirical formulas Only one is an empirical formula. HCl The subscripts cannot be reduced. What is the difference between a 6 M solution of H2SO4 and a 1 M solution of H2SO4 ? The 6 M (molar) sulfuric acid is much stronger. It has 6 moles of the acid dissolved in 1 liter of solution. The 1 M has 1 mole dissolved in 1 liter. So how many grams of H2SO4 would you need to make a 1 M solution? A 1 M solution has 1 mole dissolved in 1 liter so find the molar mass of H2SO4 H 2x1=2 S 1 x 32 = 32 O 4 x 16 = 64 That totals 98 g. A 1 M solution has 1 mole dissolved in 1 liter so add 98 grams of H2SO4 to almost one liter of water and then add enough water to make exactly 1 L. So how many grams of HCl would you need to make a 2 M solution? A 2 M solution has 2 moles dissolved in 1 liter so find the molar mass of HCl. H 1 Cl 35.5 Total = 36.5 g So how many grams of HCl would you need to make a 2 M solution? A 2 M solution has 2 moles dissolved in 1 liter so add 73 grams of HCl to almost one liter of water then add enough water to make exactly one liter of solution. This aqueous solution of HCl is named hydrochloric acid!!! A 2 M solution of HCl can do some serious damage to clothes and skin so beware. Rinse immediately and put baking soda on spills ASAP. History of the Mole The number of objects in one mole, that is, 6.02 x 1023, is commonly referred to as Avogadro's number. Amadeo Avogadro was an Italian physics professor who proposed in 1811 that equal volumes of different gases at the same temperature contain equal numbers of molecules. History of the Mole About fifty years later, an Italian scientist named Stanislao Cannizzaro used Avogadro's hypothesis to develop a set of atomic weights for the known elements by comparing the masses of equal volumes of gas. Building on this work, an Austrian high school teacher named Josef Loschmidt calculated the size of a molecule of air in 1865, and thus developed an estimate for the number of molecules in a given volume of air. While these early estimates have since been refined, they led to the concept of the mole - that is, the theory that in a defined mass of an element (its atomic weight) there is a precise number of atoms: Avogadro's number. Molar Mass A sample of any element with a mass equal to that element's atomic weight (in grams) will contain precisely one mole of atoms (6.02 x 1023 atoms). For example, helium has an atomic weight of 4.00. Therefore, 4.00 grams of helium will contain one mole of helium atoms. You can also work with fractions (or multiples) of moles: Mole challenge activity Do you know why the mole is an important unit? Here’s your chance to show your knowledge and solve a problem. The challenge: Using iron and sulfur, mass out a number of grams of each that should combine perfectly with no waste. Record both masses. Mole challenge activity Put chemicals together in a test tube. Use a magnet on the outside of the test tube to move around the iron in the inside (just to show that iron is magnetic). With a wooden splint or glass stirring rod, mix well. Using test tube holder and bunsen burner, heat the chemicals until there is a chemical reaction. Mole challenge activity Using test tube holder and bunsen burner, heat the chemicals until there is a chemical reaction. Allow substance to cool. Bring test tube to teacher who will evaluate the product (FeS). If successful, there won’t be anything magnetic (iron) or any sulfur leftover. Only FeS should exist and that compound is NOT magnetic. Test with a magnet. It may be necessary to break the test tube to extract the substance. Your teacher will do that for you. DON’T you break it. How many moles of HCl would you need to make 2 L of a 3M solution? http://www.chem1.com/acad/webt ext/intro/MOL.html

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