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MT1 Solutions

# MT1 Solutions - Chemistry 1A Spring 2010 Midterm 1...

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Chemistry 1A, Spring 2010 Midterm 1 Solutions Feb 8, 2010 Problems 1. Of the reactions below, which one is not a combination reaction? E) 2 CH 4 + 4 O 2 2 CO 2 + 4 H 2 O, because it has two products 2. Vanadium has two naturally occurring isotopes, 50 V, with an atomic mass of 49.9472 amu and 51 V, with an atomic mass of 50.9440 amu. The atomic weight of vanadium is 50.9415. The percent abundances of the vanadium isotopes are 50 V and 51 V. x m 50 V + y m 51 V = 50 . 9415 x + y = 1 y = 1 x x m 50 V + (1 x ) m 51 V = 50 . 9415 49 . 9472 x + 50 . 9440 50 . 9440 x = 50 . 9415 0 . 9968 x = 0 . 0025 x = 0 . 002508 y = 0 . 9975 (A) Vanadium is 0.25% 50 V and 99.75% 51 V 3. One moles of ? contains the largest number of atoms. B) C 10 H 8 has 18 moles of atoms 4. When the following equations is balanced, the coe cients are: (B) 4 NH 3 (g) + 7 O 2 (g) 4 NO 2 + 6 H 2 O 5. The combustion of propane (C 3 H 8 ) produces CO 2 and H 2 O. The reaction of 2.5 mole of C 3 H 8 in air will produce ? moles of H 2 O theoretically. 1 C 3 H 8 + 5 O 2 3 CO 2 + 4 H 2 O 2 . 5 mole C 3 H 8 × 4 mole H 2 O 1 mole C 3 H 8 = 10 moles H 2 O (D) 10 moles H 2 O 1

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6. A 4.000g mixture of NaCl and KCl is dissolved in water and reacted with AgNO 3 to make 8.5904 g AgCl. What is the mass percent of NaCl in the original mixture? 8 . 5904 ✘✘ g AgCl × 1 ✭✭ mol AgCl 143 . 35 ✘✘ g AgCl × 1 mol Cl 1 ✭✭ mol AgCl = 0 . 05993 mol Cl atoms We know that the total number of Cl atoms must be conserved in the reactants and the products, which we know to be 0.05993 mol Cl atoms. Therefore let’s define the following: x = number of moles Cl from NaCl 0 . 05993 x = number of moles of Cl from KCl We can now relate this to the weight of the mixture (4.000 g) via the individual molecular weights. x ✭✭ mol Cl atoms from NaCl × 1 mol NaCl 1 ✭✭ mol Cl atoms = x mol NaCl 0 . 05993 mol Cl atoms from KCl × 1 mol KCl 1 mol Cl atoms = 0 . 05993 x mol KCl x ✭✭ mol NaCl 58 . 44 g NaCl 1 ✭✭ mol NaCl + (0 . 05993 x ) mol KCl 74 . 55 g KCl 1 mol KCl = 4 . 000 g 58 . 44 x + 4 . 4675 74 . 55 x = 4 . 000 16 . 11 x = 0 . 46749 x = 0 . 02902 = mol NaCl 0 . 02902 ✭✭ mol NaCl 58 . 44 g NaCl 1 ✭✭ mol NaCl = 1 . 696 g NaCl 0 . 05993 0 . 02902 = 0 . 03091 mol KCl 74 . 55 g KCl 1 mol KCl = 2 . 304 g KCl %NaCl = 1 . 696 g 4 . 000 g × 100 = 42 . 40% %KCl = 2 . 304 g 4 . 000 g × 100 = 57 . 60% (D) 42.40% NaCl 7. ? and ? reside in the atomic nucleus? (C) protons, neutrons 8. An ion has 8 protons, 9 neutrons, and 10 electrons. The symbol for this ion is ? . (A) 17 O 2 . 8 protons makes the element oxygen, 10 electrons gives an overall -2 charge, and 9 neutrons + 8 protons gives a weight of 17.
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