Final MWF-solutions - Version 336 Final MWF sutclie(50965...

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Version 336 – Final MWF – sutcliffe – (50965) 1 This print-out should have 46 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. 001 10.0points Methane (CH 4 ) forms a molecular solid. What type of forces hold it in a solid con- figuration? I) London forces II) dipole-dipole forces III) hydrogen bonding 1. II only 2. I only correct 3. I and II only 4. III only 5. II and III only 6. I, II, and III Explanation: Solid methane is held together only by Lon- don forces. 002 10.0points Which of the following elements is not cor- rectly paired with its group (family) name? 1. Strontium (Sr), Alkaline earth metals 2. Radon (Rn), Noble gases 3. Lithium (Li), Alkali metals 4. Bismuth (Bi), Halogens correct Explanation: The pnictogens are the family containing nitrogen through bismuth. The halogens are fluorine through astatine. 003 10.0points Write the formula unit equation for this re- action occurring in water: Sodium phos- phate and calcium chloride are mixed to form sodium chloride and calcium phosphate. 1. 2 Na 3 PO 4 (aq) + 3 CaCl 2 (aq) 6 NaCl (aq) + Ca 3 (PO 4 ) 2 (s) correct 2. Na 2 PO 4 (s) + CaCl 2 (aq) 2 NaCl (aq) + CaPO 4 (s) 3. 2 Na 3 PO 4 (aq) + 3 CaCl 2 (s) 6 NaCl (aq) + Ca 3 (PO 4 ) 2 (s) 4. NaPO 4 (aq) + CaCl(aq) NaCl (aq) + CaPO 4 (s) 5. Na 2 PO 4 (aq) + CaCl 2 (aq) 2 NaCl (aq) + CaPO 4 (s) 6. 2 Na 3 PO 4 (s) + 3 CaCl 2 (aq) 6 NaCl (aq) + Ca 3 (PO 4 ) 2 (s) Explanation: 004 10.0points Calculate the change in entropy when 2.00 moles of methane are compressed isother- mally to one fifth of its original volume. Treat methane as an ideal gas. 1. - 1.39 J · K 1 2. +13.4 J · K 1 3. - 26.8 J · K 1 correct 4. +26.8 J · K 1 5. - 13.4 J · K 1 Explanation: If compressed to one fifth the original vol- ume this means V 2 = V 1 5 or V 2 V 1 = 1 5 Δ S = n R ln parenleftbigg V 2 V 1 parenrightbigg = (2 . 00 mol)(8 . 314 J · mol 1 · K 1 ) × ln parenleftbigg 1 5 parenrightbigg = - 26 . 7617 J · K 1
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Version 336 – Final MWF – sutcliffe – (50965) 2 We expect a negative answer since the volume decreased. 005 10.0points How many oxygen atoms (not molecules) are present in 1.25 moles of SO 2 ? 1. 1 . 50 × 10 24 atoms correct 2. 1.25 atoms 3. 7 . 52 × 10 23 atoms 4. 2.50 atoms Explanation: n = 1.25 mol In one mole of SO 2 , there are 2 moles of oxygen atoms. We can use this ratio to con- vert from moles of SO 2 to moles of oxygen atoms: ? mol O = 1 . 25 moles SO 2 × 2 mol O 1 mol SO 2 = 2 . 5 mol O We use Avogadro’s number to convert from moles of oxygen atoms to the number of indi- vidual oxygen atoms: ? atoms O = 2 . 5 mol O × 6 . 02 × 10 23 atoms O 1 mol O = 1 . 505 × 10 24 atoms O 006 10.0points Which of the following would probably have a positive Δ S value? 1. He(g, 2 atm) He(g, 10 atm) 2. H 2 (g) + I 2 (s) 2 HI(g) correct 3. 2 NO 2 (g) N 2 O 4 (g) 4. 2 Ag(s) + Br 2 ( ) 2 AgBr(s) 5. O 2 (g) O 2 (aq) Explanation: We can predict the sign of Δ S by noting the relative order of entropy: solids (lowest) < liquids < solutions < gases (highest) and the number of moles of each type. Also, if a gas is compressed (pressure increased) its entropy decreases.
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