exam 1-solutions-sum

# exam 1-solutions-sum - Version 103 – exam 1 – Cowley...

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Unformatted text preview: Version 103 – exam 1 – Cowley – (90225) 1 This print-out should have 25 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. 001 10.0 points The alcohol methanol (CH 3 OH) can be oxi- dized to formic acid (HCO 2 H) by oxygen: CH 3 OH + O 2 → HCO 2 H + H 2 O When 8.0 grams of the alcohol are oxidized in the presence of excess O 2 , 8.0 grams of formic acid are formed. Calculate the percent yield. 1. 70% correct 2. 12% 3. 46% 4. 100% Explanation: m CH 3 OH = 8.0 g m HCO 2 H = 8.0 g The fact that we know O 2 is excess makes this problem easier in that we know that CH 3 OH is the limiting reactant. Using the balanced equation, we have ? g HCO 2 H = 8 . 0 g CH 3 OH × parenleftbigg 1 mol CH 3 OH 32 . 042 g CH 3 OH parenrightbigg × parenleftbigg 1 mol HCO 2 H 1 mol CH 3 OH parenrightbigg × parenleftbigg 46 . 026 g HCO 2 H 1 mol HCO 2 H parenrightbigg = 11 . 5 g HCO 2 H 11.5 g is then the theoretical yield of this reac- tion. If 8.0 g of HCO 2 H is actually obtained, the percent yield is ? % yield = actual yield theoretical yield × 100% = 8 . 0 g 11 . 5 g × 100% = 70% 002 10.0 points The binary compound PCl 3 is called 1. None of these. 2. triphosphorus chloride. 3. phosphorus trichloride. correct 4. phosphorus chloride. 5. monophosphorus trichloride. Explanation: PCl 3 is a covalent compound and is there- fore named using prefixes to indicate the num- ber of each type of atom. The prefix for three is “tri” so this molecule is named phosphorous trichloride. The “mono” prefix is commonly left off of the first element name in a binary compound. 003 10.0 points Choose the formula for the compound sodium chromate. 1. Na 2 Cr 2 O 7 2. NaCrO 3 3. Na 2 Cr 4. Na 2 Cr 3 5. Na(CrO) 2 6. Na 2 CrO 4 correct 7. NaCrO 4 8. NaCr 2 O 7 Explanation: 004 10.0 points Isotopes of an element have the same number of 1. protons, but different numbers of elec- trons. Version 103 – exam 1 – Cowley – (90225) 2 2. protons, but different numbers of neu- trons. correct 3. neutrons, but different numbers of pro- tons. 4. electrons, but different numbers of pro- tons. Explanation: Definition 005 10.0 points A magnesium ion differs from a magnesium atom because a magnesium ion has 1. two less electrons. correct 2. one less electron. 3. one more electron. 4. two less neutrons. Explanation: Mg is in column 2A of the periodic table, so it has 2 valence e- . When it loses those 2 electrons, its charge is 2+. 006 10.0 points A compound containing only carbon and hy- drogen is found to contain 80.0% carbon. It has a molecular weight of 30.0. What is its molecular formula? 1. C 2 H 4 2. C 2 H 6 correct 3. C 3 H 8 4. CH 3 5. C 4 H Explanation: % C = 80% MW = 30 g Assume that we have 100 g of the com- pound and therefore, 80.0 g of carbon in the compound: ? mol C = 80 . 0 g C × 1 mol C 12 . 0 g C = 6 . 7 mol C Because the compound contains only carbon and hydrogen, we must have 20.0 g of hydro- gen in the compound.gen in the compound....
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exam 1-solutions-sum - Version 103 – exam 1 – Cowley...

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