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Chemical Formulas: The Basics

Learn the basics of chemical formulas and how to use subscripts to represent the number of atoms of each element in a molecule. Understand the rules for subscripts and coefficients, and how to count the number of atoms in a given formula. Explore examples of chemical formulas and hydrates.

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Chemical Formulas: The Basics

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  1. Chemical Formulas: The Basics

  2. Subscripts in Chemical Formulas • Numbers inside formulas. • Smaller than the font size & lowered below the line of type. • Subscripts show the number of atoms of each element in a representative unit.

  3. Rules for Subscripts in Formulas • No subscript – it’s understood to be 1. • Subscripts refer to the element immediately preceding them. • Subscripts after parentheses refer to everything inside ( ).

  4. Chemical Formulas • H2O has 2 hydrogens and 1 oxygen • NH3 has 1 nitrogen and 3 hydrogens • CH4 has ? • CO2 has ? • AgNO3 has ? • Al(NO3)3 has ? 1 carbon and 4 hydrogens 1 carbon and 2 oxygens 1 silver, 1 nitrogen, 3 oxygens 1 aluminum, 3 nitrogens, and 9 oxygens

  5. No. of atoms in formulas • Al(NO3)3 • Al = • N = • O = • Total = Subscript outside ( ) X subscript inside ( ) = # of atoms 1 1 X 3 = 3 3 X 3 = 9 13 Hint: Look for uppercase letters!

  6. No. of atoms in formulas • Mg3(PO4)2 • Mg = • P = • O = • Total = 3 2 X 1 = 2 4 X 2 = 8 13

  7. Coefficients • Numbers in front of formulas • Refer to everything in the formula immediately following • No coefficient? It’s understood to be 1.

  8. Coefficients • 2H2O means 2(H2O) • To count atoms take coefficient X subscript • 2 X 2 = 4 H’s • 2 X 1 = 2 O’s

  9. Hydrates • A group of salts that have water molecules stuffed in the empty spaces • Formulas are distinctive • Ex: CuSO45H2O •  means “is associated with” or PLUS • Not a true chemical bond, but the structure is definite so the number of empty spaces is definite and the number of water molecules is definite

  10. CuSO45H2O • Count up the atoms! 1 X Cu 1 X S 4 X O 10 X H 5 X O

  11. 5[CuSO45H2O] • Count up the atoms! Coefficients X Subscripts 5 X Cu 5 X S 20 X O 50 X H 25 X O

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