Homework 02 - Su Yung – Homework 2 – Due midnight – Inst FAKHREDDINE 1 This print-out should have 48 questions Multiple-choice questions may

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Unformatted text preview: Su, Yung – Homework 2 – Due: Sep 19 2007, midnight – Inst: FAKHREDDINE 1 This print-out should have 48 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. The due time is Central time. Each question is worth 1 point. 001 (part 1 of 1) 1 points Calculate the number of carbon atoms in 4.56 grams of ethanol (CH 3 CH 2 OH). 1. 1 . 79 × 10 23 atoms 2. 5 . 97 × 10 22 atoms 3. 1 . 19 × 10 23 atoms correct 4. 5 . 49 × 10 24 atoms 5. 2 . 53 × 10 26 atoms Explanation: m CH 3 CH 2 OH = 4.56 g Each CH 3 CH 2 OH molecule contains two carbon atoms. There are Avogadro’s number of ethanol molecules in one mole of ethanol. We need the molecular mass of ethanol so we can convert grams of ethanol to moles ethanol: Molecular mass of CH 3 CH 2 OH = 2(12 . 01 g / mol) + 6(1 . 01 g / mol) +1(16 . 00 g / mol) = 46 . 08 g / mol We can use this molecular mass to convert g ethanol to mol ethanol: ? mol ethanol = 4 . 56 g CH 3 CH 2 OH × 1 mol CH 3 CH 2 OH 46 . 08 g CH 3 CH 2 OH = 0 . 09896 mol CH 3 CH 2 OH We can now use Avogadro’s number and the ratio of C atoms to CH 3 CH 2 OH molecules to find the number of carbon atoms: ? atoms C = 0 . 09896 mol CH 3 CH 2 OH × 6 . 022 × 10 23 molec CH 3 CH 2 OH 1 mol CH 3 CH 2 OH × 2 atoms C 1 molec CH 3 CH 2 OH = 1 . 192 × 10 23 atoms C 002 (part 1 of 1) 1 points Which of the following is NOT a correct de- scription of 16.0 grams of methane (CH 4 )? 1. the amount of methane that contains 6.022 × 10 23 C atoms 2. the amount of methane that contains two molecules of H 2 correct 3. the amount of methane that contains 4 × 6 . 022 × 10 23 H atoms 4. the amount of methane that contains 12.0 g C Explanation: MW = 16 g / mol Note that the molecular weight of methane is 16 g/mol. This was calculated by adding the mass of one carbon (12 g/mol) and 4 hydrogens (4 × 1 g/mol). This means that the 16 gram sample in the problem is equivalent to a one mole sample. Therefore it is true that the sample would have Avogadro’s number of molecules (6 . 022 × 10 23 molecules/mol). Since there are four hydrogen atoms per methane molecule, calculating the number of hydrogen atoms in one mole of methane gives ? atoms H = 1 mole CH 4 × 6 . 02 × 10 23 molec CH 4 1 mol CH 4 × 4 atoms H 1 molec CH 4 = 4 × 6 . 02 × 10 23 atoms H So this answer is also true. Now, how many grams of carbon would be in 1 mole methane? ? g C = 1 mol CH 4 × 1 mol C 1 mol CH 4 Su, Yung – Homework 2 – Due: Sep 19 2007, midnight – Inst: FAKHREDDINE 2 × 12 g C 1 mol C = 12 g C This leaves the “contains two molecules H 2 ” answer as the false answer. This answer is not a correct description of 16 g methane because, while there are four hydrogen atoms, these hydrogen atoms are not bound together into diatomic hydrogen molecules....
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This note was uploaded on 04/15/2009 for the course CH 54740 taught by Professor Fakhreddine during the Spring '09 term at University of Texas at Austin.

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Homework 02 - Su Yung – Homework 2 – Due midnight – Inst FAKHREDDINE 1 This print-out should have 48 questions Multiple-choice questions may

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