Chem HWK
GgboladeSummary 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: moles ⟷ grams
· 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 atoms ⟷ moles 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
(2) How many moles of silver phosphate, Ag3PO4 are there if there are 0.13 moles of silver?
1 mole Ag3PO4 = 3 moles Ag
· 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
(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
(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
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