CHEM LAB Report

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1. Each atom present is represented by its element symbol.

2. The number of each type of atom is indicated by a subscript written to the right of the element symbol.

3. When only one atom of a given type is present, the subscript 1 is not written.

Rules for Writing Formulas

Total Number of Each Type of Atom in a Chemical Formula

• Determine the number of each type of atom in Mg(NO3)2.

• Mg(NO3)2 indicates a compound containing one magnesium atom (present as the Mg2+ ion) and two NO3− groups.

Mg: 1 = 1 Mg

N: 1 × 2 = 2 N O: 3 × 2 = 6 O

Types of Chemical Formulas • An empirical formula gives the relative number of atoms of

each element in a compound.

• A molecular formula gives the actual number of atoms of each element in a molecule of the compound.

• For example, the molecular formula for hydrogen peroxide is H2O2, and its empirical formula is HO.

• The molecular formula is always a whole number multiple of the empirical formula.

Molar Mass Of Elements

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Converting between Grams and Moles Calculate the number of moles of sulfur in 57.8 g of sulfur.

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• Mass in grams of one mole of the substance: Molar Mass of N = 14.01 g/mol

Molar Mass of H2O = 18.02 g/mol (2 × 1.008 g) + 16.00 g

Molar Mass of Ba(NO3)2 = 261.35 g/mol 137.33 g + (2 × 14.01 g) + (6 × 16.00 g)

Counting Molecules by the Gram Molar Mass

Molar Mass of K3PO4

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Mass Percent Composition of Compounds

• The mass percent composition, or mass percent, of an element is the element’s percentage of the total mass of the compound.

Finding Mass Percent Composition

• A 0.358-g sample of chromium reacts with oxygen to form 0.523 g of the metal oxide.

• The mass percent of chromium is: