PS 05 2019 key - chem PDF

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Problem(Set(05(–(Atomic(Mass(and(the(Periodic(Table! Chem!105! 1. (a) 2.46 Naturally occurring sulfur consists of four isotopes: 32S (31.9721 amu, 95.04%); 33S (32.9715 amu, 0.75%); 34S (33.9679 amu, 4.20%); and 36S (35.9671 amu, 0.01%). Use this information to calculate the average atomic mass of sulfur. (.9504 x 31.9721 amu) + (.0075 x 32.9715 amu) + (.042 x 33.9679 amu) + (.0001 x 35.9671 amu) = = 32.1 amu (if you did not round at each step) = (30.63 amu) + (0.25 amu) + (1.4 amu) + (0.004 amu) = 32.3 amu (if you did round at each step) (b) Calculate the atomic weight of nickel to the correct number of significant figures using the following data: 58 Ni, 68.075%, 57.9353429 amu à (.0.68075 x 57.9353429 amu) = 39.439 amu 60 Ni, 26.221%, 59.9307864 amu à + (0.26221 x 59.9307864 amu) = + 15.714 amu 61 Ni, 1.1399%, 60.9310560 amu à + (0.011399 x 60.9310560 amu) = + 0.69455 amu 62 Ni, 3.637%, 61.9283451 amu à + (0.03637 x 61.9283451 amu) = + 2.252 amu 64 Ni, 0.9271%, 63.9279660 amu à + (0.009271 x 63.9279660 amu) = + 0.5927 amu = 58.692 amu (or 58.693 if you didn’t round in intermediate steps) In both (a) and (b), the last digit is clearly the last ‘significant’ digit because it varies depending on how the calculations are rounded. In each case there are certain, known digits + 1 uncertain digit 2. (a) Naturally occurring Mg has three stable isotopes, 24Mg, 25Mg, and 26Mg. This means that there are three forms of Mg atoms, each with a unique atomic weight. This appears to be contrary to the idea that atomic weight is the defining property of an element. Explain (in 3 sentences or less) why all three of these types of atoms are indeed the same element. It is the atomic number, Z (the number of protons in the nucleus) that defines an element, not the atomic weight. The atomic weight includes the mass of any neutrons in the nucleus, and the number of neutrons can vary between atoms having the same Z; this is the definition of an isotope – atoms with the same Z but different numbers of neutrons. We consider them to be the same element because they exhibit the same properties & reactivity despite having different numbers of neutrons. (b) Chemical analyses conducted by the first Mars rover robotic vehicle in its 1997 mission produced the magnesium isotope data shown in the table below. Is the average atomic mass of magnesium in this Martian sample the same as on Earth (24.305 amu)? (use the correct number of sig figs): Isotope Mass (amu) Natural Abundance (%) 24 Mg 23.9850 78.70 25 Mg 24.9858 10.13 26 Mg 25.9826 11.17 mMg = (0.7870 ´ 23.9850 amu) + (0.1013 ´ 24.9858 amu) + (0.1117 ´ 25.9826 amu) = 24.31 amu The average mass of Mg on Mars is the same as here on Earth. The mass of Mg on Mars should be close to the same value as on Earth; the magnesium on both planets arrived in the solar system via the same ancient stardust. 3. The following elements all have two stable isotopes. Which isotope in each of the following pairs of stable isotopes is more abundant? Explain how you know. Looking at the average atomic masses of the elements given on the periodic table, we can deduce a) 10B or 11B is more abundant; the average atomic weight of B is 10.811 amu, so there must be more 11B, pulling the average closer to 11 amu than to 10 amu

b) 6Li or 7Li; c) 14N or 15N d) 20Ne or 22Ne e) 35Cl or 37Cl 4. Which of the highlighted elements below form monatomic ions with a charge of (a) 1+, (b) 2+, (c) 3+, (d) 1– and (e) 2–? Highlighted elements from left to right are K, Mg, Sc, Ag, O, and I. + (a) Elements in group 1 form 1+ ions, so K will form K (dark blue). Silver (green) also typically forms a 1+ cation. 2+ (b) Elements in group 2 form 2+ ions, so Mg forms Mg (gray). 3+ (c) Elements in group 3 form 3+ ions, so Sc forms Sc (yellow). – (d) Elements in group 17 (the halogens) form 1– ions, so I forms I (purple). 2– (e) Elements in group 16 form 2– ions, so O forms O (red).

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5. Describe how the charges of the monatomic ions that elements form change as group number increases in a particular row of the periodic table and how ion charges change as the row number increases in a particular group. As we proceed across a row in the periodic table and the group number increases, we first start off with a low 1+ charge for the alkali metals but then increase more positively by 1 through the alkaline earth, boron, and carbon groups. Then the trend shifts to highly negative charges, as in 3– for nitrogen and phosphorus and then reducing one unit through oxygen (2–) and fluorine (1–) to zero for neon. As we proceed down a group on the periodic table, the charge on the monatomic ion stays the same except for the heavy group 13, 14, and (not shown in Figure 2.10) 15 elements, which show two charges: one like the rest of its lighter group members and the other two units less positive....