CHEM 105 1st Edition Lecture 1Outline of Last Lecture • Section 1.4 intensive vs extensive properties• Section 1.8 precision vs. accuracy• Section 1.10 absolute zero and the Kelvin temperature scale• Systems tend to change in a way that lowers their potential energy• Ligh potential energy = unstable• Low potential energy = stableOutline of Current Lecture • Average Atomic Mass • To understand the meaning of a weighted average, consider the masses and natural abundances of the three istopes of neon in the table to the left. • Calculating an Average Atomic Mass Current LecturePay close attention to what your text says about reading measuring devices and using significantfigures (pages 21-24). Rule 3 on page 22 is wishy-washy. Zeros at the end of a value that contains no decimal point are not significant!Don’t worry about significant figures until after you have a final answer. NEVER round numbers in the middle of a calculation!The rules for addition/subtraction are different from the rules for multiplication/division.Complications arise in problems that include both types of operations.-Rounding rules (at the end of the calculation)-If the first digit dropped is 0-4, round down.-If the first digit dropped is 5-9, round up.Q: A measuring cylinder contains 35.43mL of water. If a metal object with a mass of 82.502g is placed in the cylinder and water, the water level rises to 42.07mL. What is the density of the metal?Answer: D=M/V 35.43ml-42.07ml=6.64ml D= 82.502g/6.64mlD=12.4g/ml Dimensional AnalysisConvert 14.757 pounds per square inch to kg per square centimeter.2.54cm=1 inch (exact!)1 kg = 2.205 lbs (not exact)-Magnesium has an atomic mass of 24.3050 amu. There are three stable isotopes: 78.70% of all magnesium atoms have a mass of 23.985042 amu.10.13% of all magnesium atoms have a mass of 24.98584 amu.Q: Identify the mass number, the natural abundance, and the isotopic mass of the remaining stable isotope.Answer: 78.70%+10.13%= 88.83%100.00% - 88.83%= 11.17% natural abundance 0.7870 X 23.985042amu + 0.1013 X 24.98584amu + 0.1117x =24.3050amu18.876228amu +2.53106559amu+ 0.117x =24.3050amu0.1117x =24.3050amu - 18.876228amu - 2.53106559amux= 2.8977064amu/0.1117 = 25.94186578amu ~ 25.9 amu This would be the mass 26 -Gallium consists of two naturally occurring isotopes: 69Ga has an isotopic mass of 68.9257 amu 71Ga has an isotopic mass of 70.9249 amuThe atomic mass of gallium is 69.723 amuQ: What are the %abundances of the two isotopes?Answer: If the abundances of 69 Ga is x%, the abundances of 71Ga is (100.00-x)% x X68.9259amu+(100.00%-x) X70.9249amu/100.00% =69.72368.9257amuX x+7092.49amu X%-70.9249amu X x =6972.3amu -1.9992amu X x= -120.19amu%X= -120.19amu% /-1.9992amu =60.11905% - 100.00%= 39.88% 69Ga is 60.12%71Ga is
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