Chemical Formulas Formula Mass Molar Mass Subscripts in
Chemical Formulas: Formula Mass & Molar Mass
Subscripts in Chemical Formulas n n subscripts show # atoms of each element in representative unit subscripts after parentheses refer to everything inside ( )
Molar mass of an element n molar mass is: mass (in grams) of 1 mole of any pure substance n numerically equal to atomic mass (rounded to whole number), but … n n units change from amu to grams per mole (g/mol)
Molar mass of elements n Na has atomic mass of 22. 98977 amu rounds to 23 amu n molar mass = 23 grams/mol n n K has atomic mass of 39. 0983 amu rounds to 39 amu n molar mass = 39. 0983 grams/mol n n Ca has atomic mass of 40. 08 amu rounds to 40 amu n molar mass = 40. 08 grams/mol n
Molar Mass n n MOLAR mass: always refers to mass in grams per mole (g/mol) formula mass (FM) or gram formula mass (GFM): refers to mass in grams (g) n generally used for amounts other than one mole
Mole to Mass conversions n use PT for conversions involving mass unless you want to memorize the masses of all the elements!!
Molar Mass of Compounds equal to sum of masses of all atoms in compound's formula round atomic masses to whole number ex: H 2 O n 2 H’s = 2 x 1 g = 2 g n 1 O = 1 x 16 g = 16 g n n FM of H 2 O is sum: 2 g + 16 g = 18 g n molar mass of H 2 O is 18 g/mol
CH 4 n 1 C = 1 x 12 g = 12 g n 4 H’s = 4 x 1 g = 4 g n FM is sum = 4 g + 12 g = 16 g n molar mass is 16 g/mol
n challenge: find FM of Cu. SO 4 • 5 H 2 O 1(64) + 1(32) + 4(16) + 10(1) + 5(16) = 250 g 4 Ba 3(PO 4)2 12(137) + 8(31) + 32(16) = 2, 404 g OR 3(137) + 2(31) + 8(16) = 601 g X 4 =2, 404 g
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