The Mole Concept
the extreme smallness of atoms and molecules makes it necessary to define a very large “standard sample”
it is referred to as the MOLE
it’s the SI base unit for amount of chemical substance
one mole consists of 6.02 x 1023 atoms or molecules; it is known as Avogadro’s Number
the mass of one mole of an element is the same as the atomic mass of the element
one mole of bromine, atomic mass 79.90 amu, has a mass of 79.90 g; this is called the molar mass
the molar mass of chromium,
Cr, is 52.00 g/mol.
types of calculations:
(i) mass of 1.50 moles of chromium
(ii) # of moles in 5.00g of O2
(iii) determining the molar mass of a compound; what is the molar mass of CuSO4?
(iv) # of moles given a particular mass of a compound; how many moles is 56.00 g of NaCl?
(v) mass of a compound given a particular # of moles; determine the mass of 0.1 moles of H2SO4.
1. How many grams of sulfur are present in 0.150 mol of Au?
1 mole of Au → 197.0 g
0.150 mol of Au→ x g
x = 197.0 g x 0.150 moles = 29.6 g Au
2. How many atoms are in a sample of uranium with a mass of 1.00 x 10–6 g ?
1 mol U → 238.0 g U
1 mol U → 2.03 x 1023 atoms
238.0 g U → 2.03 x 1023 atoms
1.00 x 10–6 g U → x atoms
x = (2.03 x 1023 atoms)( 1.00 x 10–6 g) = 2.53 x 1015 atoms
3. How many grams is 0.115 mol of Ca3(PO4)2 ?
1 mol of Ca3(PO4)2 → 310.18 g
0.115 mol → x g
x = 310.18 g x 0.115 mol = 35.7 g Ca3(PO4)2
4. How many grams of Cl are needed to combine with 24.4 g of Si to make silicon tetrachloride, SiCl4?
The balanced equation for the reaction must be written
Si(s) + 2Cl2(g) → SiCl4(s)
From the equation we see that:
1 mol Si reacts with 2 mol of Cl2 1 mol Si = 28.09 g
x mol Si = 24.4 g
0.869 mol Si reacts with x mol Cl2
x = 1 mol x 24.4 g = 0.869 mol Si
x mol Cl2 = 2 mol Cl2 x 0.869 mol Si 24.4 g
1 mol Si use this value in your calculations
= 1.74 mol
1 mol Cl2 = 70.90 g
1.74 mol Cl2 = x g
x g Cl2 = 70.90 g x 1.74 mol
= 123.366 g = 123 g Cl2
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