Frequently Asked Question

Here's how to calculate the moles of each substance, step-by-step, including molar mass calculations and unit cancellation:
a) 2.50 g of lead (Pb)
1. Molar Mass of Lead (Pb): The molar mass of lead is found on the periodic table: 207.2 g/mol.
2. Conversion:

2.50 g Pb x (1 mol Pb / 207.2 g Pb) = 0.0121 mol Pb

b) 5.00 g of KBr
1. Molar Mass of KBr:

Potassium (K): 39.10 g/mol

Bromine (Br): 79.90 g/mol

Total molar mass: 39.10 g/mol + 79.90 g/mol = 119.00 g/mol
2. Conversion:

5.00 g KBr x (1 mol KBr / 119.00 g KBr) = 0.0420 mol KBr

c) 3.75 g of Ca₃(PO₄)₂
1. Molar Mass of Ca₃(PO₄)₂:

Calcium (Ca): 40.08 g/mol (x3 = 120.24 g/mol)

Phosphorus (P): 30.97 g/mol (x2 = 61.94 g/mol)

Oxygen (O): 16.00 g/mol (x8 = 128.00 g/mol)

Total molar mass: 120.24 g/mol + 61.94 g/mol + 128.00 g/mol = 310.18 g/mol
2. Conversion:

3.75 g Ca₃(PO₄)₂ x (1 mol Ca₃(PO₄)₂ / 310.18 g Ca₃(PO₄)₂) = 0.0121 mol Ca₃(PO₄)₂

d) 10.00 g of CH₂CH₂CH₂CH₂CH₂CH₃ (Hexane)
1. Molar Mass of Hexane:

Carbon (C): 12.01 g/mol (x6 = 72.06 g/mol)

Hydrogen (H): 1.01 g/mol (x14 = 14.14 g/mol)

Total molar mass: 72.06 g/mol + 14.14 g/mol = 86.20 g/mol
2. Conversion:

10.00 g C₆H₁₄ x (1 mol C₆H₁₄ / 86.20 g C₆H₁₄) = 0.116 mol C₆H₁₄

e) 15.00 g of CuSO₄·5H₂O (Copper(II) Sulfate Pentahydrate)
1. Molar Mass of CuSO₄·5H₂O:

Copper (Cu): 63.55 g/mol

Sulfur (S): 32.07 g/mol

Oxygen (O): 16.00 g/mol (x4 + 5 = 9 = 144.00 g/mol)

Hydrogen (H): 1.01 g/mol (x10 = 10.10 g/mol)

Total molar mass: 63.55 g/mol + 32.07 g/mol + 144.00 g/mol + 10.10 g/mol = 249.72 g/mol
2. Conversion:

15.00 g CuSO₄·5H₂O x (1 mol CuSO₄·5H₂O / 249.72 g CuSO₄·5H₂O) = 0.0601 mol CuSO₄·5H₂O

f) a cube of copper metal with an edge length of 1.00 in. The density of copper is 8.92 g/cm³
1. Volume of the Cube:

Convert inches to centimeters: 1.00 in x (2.54 cm/in) = 2.54 cm

Volume of a cube: (edge length)³ = (2.54 cm)³ = 16.4 cm³
2. Mass of Copper:

Density = mass/volume, so mass = density x volume

Mass = (8.92 g/cm³) x (16.4 cm³) = 146 g
3. Molar Mass of Copper (Cu): 63.55 g/mol
4. Conversion:

146 g Cu x (1 mol Cu / 63.55 g Cu) = 2.30 mol Cu