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MoleTools2.docx

Summary of Mole Calculation Tools

1. Avogadro’s Number

· What it’s used for: moles# of items

· Value: 6.022x1023 items = 1 mole

· Examples:

· molecular compound – water: 1 mole H2O = 6.022 x1023 H2O molecules

· monatomic element – helium: 1 mole He = 6.022 x1023 He atoms

· diatomic element – hydrogen: 1 mole H2 = 6.022 x1023 H2 molecules

· ionic compound – calcium chloride: 1 mole CaCl2 = 6.022 x1023 CaCl2 formula units

2. Molar Mass

· What it’s used for: molesgrams

· Value: Add up atomic masses from the periodic table for all atoms in the formula.

· Examples:

· H2O

molar mass = 2(1.008) + 1(16.00) = 18.02 g/mol

⇒ 18.02 g H2O = 1 mol H2O

· Ca3(PO4)2

molar mass = 3(40.08) + 2(30.97) + 8(16.00) = 310.18 g/mol

⇒ 310.18 g Ca3(PO4)2 = 1 mol Ca3(PO4)2

3. Subscripts in a Formula

· What it’s used for: moles of atomsmoles of molecule or formula unit

· Examples:

· H2O: ⇒ 1 mole H2O = 2 moles H

⇒ 1 mole H2O = 1 mole O

· Ca3(PO4)2: ⇒ 1 mole Ca3(PO4)2 = 3 moles Ca

⇒ 1 mole Ca3(PO4)2 = 2 moles P

⇒ 1 mole Ca3(PO4)2 = 8 moles O

MOLE CALCULATIONS

· Using Subscripts in Chemical Formulas to Relate Moles

The subscripts in a chemical formula, like P4O10 represent how many atoms of each element are in one molecule.

But, they also represent the number of MOLES of each kind of atom in one MOLE of molecules.

In this example, we could write two conversion-factor statements:

1 mole of P4O10 = 4 moles of P and 1 mole of P4O10 = 10 moles of O

When subscripts are used in a calculation, they are considered to be exact numbers, so the significant figures for the answer is determined by the other numbers in the calculation.

· Examples: Using Subscripts to Relate Moles of Element to Moles of Compound

(1) How many moles of hydrogen atoms are in 6.455 moles of water?

water is H2O so 1 mole H2O = 2 moles H

6.455 mol H2O ⨯ = 12.91 mol H (4 SF)

(2) How many moles of silver phosphate, Ag3PO4 are there if there are 0.13 moles of silver?

1 mole Ag3PO4 = 3 moles Ag

0.13 mol Ag ⨯ = 0.043 mol Ag3PO4 (2 SF)

· Examples: Using Two Mole Tools in One Problem

(3) How many nitrogen atoms are there in 7.05 moles of N2O5 ?

Starting unit: moles N2O5 Ending unit: N atoms

Tools: Formula subscript for moles N2O5 to moles N

Avogadro’s number for moles N to N atoms

7.05 mol N2O5 ⨯ ⨯ = 8.49 1024 N atoms (3 SF)

(4) How many dinitrogen pentoxide molecules are there in 675 grams of N2O5?

Starting unit: grams N2O5 Ending unit: N2O5 molecules

Tools: Molar mass of N2O5 for grams N2O5 to moles N2O5

Avogadro’s number for moles N2O5 to N2O5 molecules

molar mass N2O5 = 2(14.01) + 5(16.00) = 108.02 g/mol

375 g N2O5 ⨯ ⨯ = 2.09 1024 N 2O5 molecules (3 SF)

(5) What is the mass (grams) of 1.5⨯1019 copper atoms ?

Starting unit: Cu atoms Ending unit: grams Cu

Tools: Avogadro’s number for Cu atoms to moles Cu

Molar mass of Cu for mole Cu to grams Cu

molar mass Cu = 63.55 g/mol

1.5⨯1019 Cu atoms ⨯ ⨯ = 0.0016 g Cu (2 SF)

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