Chapter 10 3/7/2017. Avogadro s Number and the Mole. Why don t we count other things by the mole?

Size: px

Start display at page:

Download "Chapter 10 3/7/2017. Avogadro s Number and the Mole. Why don t we count other things by the mole?"

Dennis Hensley
5 years ago
Views:

Avogadro s Number and the Mole Chapter 10 Many words

tiny, that counting them by the dozen would be

02 x 10 23 particles (called Avogadro s # ) Defined

Why don t we count other things by the mole?

July 2013, the amount of cash in the form of U.S.

1,200,000,000,000 dollars Is that a mole?

602,000,000,000,000,000,000,000 particles in it!

happen if you were to gather a mole (unit of

1 Avogadro s Number and the Mole Chapter 10 Many words represent a quantity Chemical Quantities Atoms are so tiny, that counting them by the dozen would be ridiculous! Chemists count by the mole. 1 mole = 6.02 x particles (called Avogadro s # ) Defined as the # of atoms in 12 g of 12 C. Why don t we count other things by the mole? According to the Federal Reserve Bank of NY, as of July 2013, the amount of cash in the form of U.S. currency (paper & coins) was 1.2 trillion. 1,200,000,000,000 dollars Is that a mole? Water Each of these beakers contains one mole of the substance. In other words, each beaker has 602,000,000,000,000,000,000,000 particles in it!! How long would it take you to count to a mole of dollars? (assume you can say one # every second) What would happen if you were to gather a mole (unit of measurement) of moles (the small furry critter) in one place? What If? Things get a big gruesome. "One mole" is close to the number of atoms in a gram of hydrogen. It s also, by chance, a decent ballpark guess for the number of grains of sand on Earth. 1

2 A mole is also a type of burrowing mammal. There are a handful of types of moles, and some of them are truly horrifying. So what would a mole of moles 602,214,129,000,000,000,000,000 animals look like? An eastern mole (Scalopusaquaticus) weighs about 75 grams, which means a mole of moles weighs ( ) 75g kg That s a little over half the mass of our moon. Mammals are largely water. A kilogram of water takes up a liter of volume, so if the moles weigh kilograms, they take up about liters of volume. (You might notice that we re ignoring the pockets of space between the moles. In a moment, you ll see why.) If these moles were released onto the Earth s surface, they d fill it up to 80 kilometers deep just about to the (former) edge of space: The cube root of liters is 3,562 kilometers, which means we re talking about a sphere with a radius of 2,210 kilometers, or a cube 2,213 miles on each edge. This smothering ocean of high-pressure meat would wipe out most life on the planet. Instead, let s gather the moles in interplanetary space. Gravitational attraction would pull them into a sphere. Meat doesn t compress very well, so it would only undergo a little bit of gravitational contraction, and we d end up with a mole planet a bit larger than the moon. The moles would have a surface gravity about one-sixteenth as strong as Earth s similar to that of Pluto. The planet would start off uniformly lukewarm probably a bit over room temperature and the gravitational contraction would heat the deep interior by a handful of degrees. But this is where it gets weird. 2

3 The mole planet is now a giant sphere of meat. It has a lot of latent energy (there are enough calories in the mole planet to support the Earth s current population for 30 billion years). Normally, when organic matter decomposes, it releases much of that energy as heat. But throughout the majority of the planet s interior, the pressure is over a hundred megapascals, which is enough to kill all bacteria and sterilize the mole remains leaving no microorganisms to break down the mole tissues. Closer to the surface, where the pressure is lower, there s another obstacle to decomposition the interior of a mole planet is low in oxygen. Without oxygen, the usual decomposition doesn t happen, and the only bacteria that can break down the moles are those which don t require oxygen. While inefficient, this anaerobic decomposition can unlock quite a bit of heat. If continued unchecked, it would heat the planet to a boil. But the decomposition is self-limiting. Few bacteria can survive at temperatures above about 60 C, so as the temperature goes up, the bacteria die off, and the decomposition slows. Throughout the planet, the mole bodies gradually break down into kerogen, a mush of organic matter which would if the planet were hotter eventually form oil. The outer surface of the planet radiates heat into space and freezes. Because the moles form a literal fur coat, when frozen it insulates the interior of the planet and slows the loss of heat to space. However, the flow of heat in the liquid interior is dominated by convection. Plumes of hot meat and bubbles of trapped gases like methane along with the air from the lungs of the deceased moles periodically rise through the mole crust and erupt volcanically from the surface, a geyser of death blasting mole bodies free of the planet. Eventually, after centuries or millennia of turmoil, the planet calms and cools enough that it begins to freeze all the way through. The deep interior is under such high pressure that as it cools, the water crystallizes out intoexotic forms of ice Molar Mass The mass in grams of one mole of a compound. The mass of one mole of any element on the periodic table is equal to its mass in grams: (He = g/mol) Example: Calculate the molar mass of NH 3 3

Calculate the molar mass of the following: NiCl 2 The Mole is the Central Unit in Chemistry The Molar Highway CaCO 3 (NH 4 ) 2 SO 4 Make Your Own

MOLES TO MASS MASS TO MOLES grams 1mole moles x = grams grams x = moles 1mole grams Example: Convert 5.0 g of Fe to moles. Example: Convert 5.0 moles of water to grams.

Use Avogadro s Number as the conversion factor. MOLES TO PARTICLES Particle can be atoms, molecules, or formula units. PARTICLES TO MOLES 23 6.

4 Calculate the molar mass of the following: NiCl 2 The Mole is the Central Unit in Chemistry The Molar Highway CaCO 3 (NH 4 ) 2 SO 4 Make Your Own Molar Highway Converting Between Mass & Moles Use dimensional analysis. Use molar mass of substance as the conversion factor. MOLES TO MASS MASS TO MOLES grams 1mole moles x = grams grams x = moles 1mole grams Example: Convert 5.0 g of Fe to moles. Example: Convert 5.0 moles of water to grams grams Sr(OH) 2 to moles 2.50 moles of Na 2 S to grams. Converting Between Particles & Moles Use dimensional analysis. Use Avogadro s Number as the conversion factor. MOLES TO PARTICLES Particle can be atoms, molecules, or formula units. PARTICLES TO MOLES X10 particles 1mole moles x = particles particles x = moles 23 1mole 6.022x10 particles Example: Convert 5.5 x atoms of C to moles. Example: Convert 7.5 moles of CH 4 to molecules. 4

5 3.20 x molecules of water to moles 92.3 moles of C to atoms. Converting Between Volume & Moles Use molar volume as the conversion factor. At standard temperature and pressure (STP) 0 o C & 1 atm, 1 mole of any gas occupies 22.4 L. MOLES TO VOLUME VOLUME TO MOLES 22.4L 1mole moles x = liters liters x = liters 1mole 22.4L Example: Convert 82 moles of CO 2 to volume. Example: Convert 9.3 liters of H 2 O to moles. 925 L of He (at STP) to moles Multi-step Conversions Mass particles Volume mass 6.67 moles of CO 2 (at STP) to L Particles volume Follow the molar highway & use dimensional analysis to solve. 487 L of CH 4 (at STP) to molecules 2.5 x particles Na 2 CO 3 to grams Percent Composition % by mass of each element in a compound. How to calculate: totalmass of element incompound % of element = x100% mass of compound Example: Calculate the percent composition of C and O in carbon monoxide and carbon dioxide. 5

Calculate the percentage composition of hydrogen and oxygen in: H 2 O H 2 O 2 Empirical Formulas The mole allows us to calculate the empirical formula for a substance given the mass % of its elements.

$If it looks like a fraction, like ¼, ½, etc, then multiply until they become a whole #) Empirical Formulas Percent to mass, mass to mole, divide by small, multiply til whole.$

6 Calculate the percentage composition of hydrogen and oxygen in: H 2 O H 2 O 2 Empirical Formulas The mole allows us to calculate the empirical formula for a substance given the mass % of its elements. Here s how 1. Base calculation on 100 grams of compound. 2. Determine moles of each element in 100 grams of compound. 3. Divide each value of moles by the smallest of the values. 4. Multiply each number by an integer to obtain all whole numbers. (General rule: If it is close to a whole #, just round it. If it looks like a fraction, like ¼, ½, etc, then multiply until they become a whole #) Empirical Formulas Percent to mass, mass to mole, divide by small, multiply til whole. Example Percent to mass, mass to mole, divide by small, multiply til whole. One of the most commonly used white pigments in paint is a compound of titanium and oxygen that contains 59.9% Ti by mass. Determine the empirical formula of this compound. Vitamin C (ascorbic acid) contains 40.92% C, 4.58% H, and 54.50% O by mass. What is the empirical formula of vitamin C? Determining Molecular Formulas Follow the same steps in determining the empirical formula Compare empirical formula s mass to molar mass given in the problem (How many times bigger is it?) Multiply empirical formula by the appropriate integer 6

7 Example A compound that contains only nitrogen and oxygen is 30.4% N by mass; the molar mass of the compound is 92 g/mol. What is the molecular formula of the compound? Ethylene glycol, the substance used in automobile antifreeze, is composed of 38.7% C, 9.7% H, and 51.6% O by mass. Its molar mass is 62.1 g/mol. What is the empirical formula? What is the molecular formula? 7

Chapter 10 Chemical Quantities

Chapter 10 Chemical Quantities 10.1 The Mole: A Measurement of Matter OBJECTIVES: Describe methods of measuring the amount of something. Define Avogadro s number as it relates to a mole of a substance.