CHEM_1017_ch_3 - Mass Relationships in Chemical Reactions...

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Mass Relationships in Chemical Reactions Atoms are extremely small objects – too small to “weigh” one at a time. Earlier workers knew that is was possible to determine the relative mass of one element compared to another. They assigned a relative mass of 1 to the lightest known element (hydrogen), and measured all other masses relative to that. This relative weight unit was called the a.m.u. – the atomic mass unit. C atom is 12 times heavier – 12 a.m.u. O atom is 16 times heavier – 16 a.m.u. U atom is 238 times heavier – 238 a.m.u.
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Scientists usually work in the metric system. They were curious to know how many hydrogen atoms (the lightest particles known) it would take in order to weigh a single gram. The number was so large, that they had to come up with a new term to talk about such a large number of objects. Words Used to Quantify Groups of Objects: A pair of dice 2 A barber shop quartet 4 A dozen eggs 12 A gross of Mardi Gras beads 144 They chose the word “mole” to represent the number of hydrogen atoms it would take to weigh 1 gram. Unfortunate choice of words, because it sounds like molecule, and some people confuse the two.
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One mole of anything is this many!!! 602,213,670,000,000,000,000,000 Called Avogadro’s Number For ease of writing, it is usually expressed in scientific notation. 6.02 x 10 23
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A single atom of hydrogen weighs 1 a.m.u. A whole mole of hydrogen atoms (i.e. Avogadro’s number of hydrogen atoms) weighs 1 gram. On a relative basis, a carbon atom is 12 times heavier than a hydrogen atom. A single C atom weighs 12 a.m.u. A mole of C atoms (6.02 x 10 23 of them) weighs 12 grams A single U atoms weighs 238 a.m.u A mole of U atoms weighs 238 grams. A mole of H atoms and a mole of C atoms, and a mole of U atoms all contain the same number of particles , but the masses are different because the masses of the individual atoms are different.
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Because hydrogen is a gas and so light, it’s difficult to weigh accurately. Later the standard of mass was changed from H to C. 1 a.m.u. = 1/12 the mass of a C atom. Every element has several isotopes. Average atomic mass is the mass of each isotope multiplied by its natural abundance. Copper metal has two stable isotopes: 69% 31% (0.69 x 63) + (0.31 x 65) = 63.62 63 29 Cu 65 29 Cu
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A mole of C atoms would contain Avogadro’s Number of C atoms (6.02 x 10 23 ) and would weigh 12 grams. Two moles of C atoms would contain twice as many particles and weigh twice as much. 2 x 6.02 x 10 23 = 1.20 x 10 24 atoms 2 x 12 = 24 grams Suppose you has 100 grams of C. 1. How many moles of C would you have? 2. How many individual C atoms would be present?
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divide by mol. wt. mult. by Av. No (6.02 x 10 23 ) grams moles particles mult. by mol. wt. divide by Av. No. 100g C
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This note was uploaded on 10/13/2010 for the course CHEM 101 taught by Professor Crago during the Spring '06 term at Loyola New Orleans.

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CHEM_1017_ch_3 - Mass Relationships in Chemical Reactions...

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