IMPORTANCE OF SIG FIGS We are always limited by the instrument technique conditions that we use to make measurements WEIGHING COMPOUNDS EX If a pharmacist wanted to weigh out 1 mg of an expensive medicine product what balance should she or he use RULES FOR SIG FIGS All nonzero integers are significant EX 5 469 4 sig figs All zeros to the left of the first nonzero digit are NOT significant these are called place holders EX 0 0045 2 sig figs All zeros between nonzero digits ARE significant EX 0 09005 4 sig figs All zeros at the end of a number that has a decimal point are significant EX 56 7800 6 sig figs Zeros at the end of a whole number may or may not be significant EX 500 500 0 5 0x10 2 5 00x10 2 1 2 or 3 depending how its written For exact numbers there is no uncertainty as to its value EX 210 students all digits significant MATHEMATICAL OPERATIONS Addition and subtraction Units must match Exponent in scientific notation must match EX 5 628 x 10 2 3 2 x 10 2 ANS 8 8 x 10 2 Multiplication and Division Goes by LIMITER EX 4809 x 45 5 x 300 0 6 56 x 10 7 SF 4 3 4 IN MIXED CASES ONLY ROUND AT THE END PERIODIC TABLE 92 naturally occurring elements In order of increasing atomic number protons Grouped by properties DIATOMIC hydrogen nitrogen oxygen fluorine chlorine bromine iodine Atomic Mass weighted average mass of one atom SEC 2 6 Atoms have mass and we need to know their mass to weigh out a specific number of atoms very small number 1 atom of carbon wigs 1 993 x 10 26 kg kg is not convenient when talking about an atom s mass And we could never weigh out a single atom anyway For convenience Chemists use the expression ATOMIC MASS UNITS amu amu is the approx weight of a proton or a neutron EX 1 carbon 12 amu exact ISOTOPIC MASS the mass of a specific atom isotope in amu remember that elements can have several isotopes ATOMIC MASS the weighted average of isotopic masses EX LITHIUM OCCURS NATURALLY AS A MIXTURE OF 6 3 Li 7 5 AND 7 3 Li 92 5 WHAT IS ITS ATOMIC MASS ANS 6 3 Li 0 075 x 6 0151 amu 0 451 amu 7 3 Li 0 925 x 7 0160 amu 6 490 amu CALCULATE AVERAGE OF ISOTOPES FOR ATOMIC MASS AVOGADRO S NUMBER We cannot count atoms or molecules We often use larger quantity expression to describe what we need In chemistry a mole is used a unit of measurement Fundamental SI unit abbrev is mol One mole of any element contains 6 022 x 10 23 molecules or atoms Number moles n Molar Mass M Mass m n M A
View Full Document
Unlocking...