Practice Problems: Moles (Answer Key)

1. How many moles are in the following:
   
   a. 1.29 x 10²⁴ hydrogen atoms in HF      2.14 moles H atoms
   
   b. 7.36 x 10²⁴ free oxygen atoms       12.2 moles O atoms
   
   c. 3.28 x 10²³ Na atoms in salt (NaCl)      0.545 moles Na atoms
   
2. How many atoms are present in the following?
   
   a. Hydrogen atoms in 3.14 mol of H₂O      3.78 x 10²⁴ H atoms
   
   b. Nitrogen atoms in 2.79 mol of NH₃       1.68 x 10²⁴ N atoms
   
   c. Oxygen atoms in 4.90 mol in Al₂O₃      8.85 x 10²⁴ O atoms
   
3. How many atoms are in a 3.56 g sample of Cu? Is this a mol of Cu?  3.37 x 10²² Cu atoms
   
4. What mass of Ni has as many atoms as there are N atoms in 63.0 g of NO₂? As Na atoms in 63.0 g of NaCl?  80.4 g Ni, 63.3 g Ni
   
5. How many g of CaCO₃ are present in a sample if there are 4.52 x 10²⁴ atoms of carbon in that sample?  751 g CaCO₃
   
6. How many molecules of HCl are in 4.91 L of HCl acid at 25�C if the density equals 1.096 g/mL?  8.89 x 10²⁵ molecules
   
7. If a 1.000 gallon (3.785 L) bottle were filled with bleach (NaClO), how many Cl atoms would be in the bottle? r_bleach = 1.090 g/mL
   3.34 x 10²⁵ Cl atoms
   
8. At 25º C, the density of water is 0.997 g/ml. At -10º C, the density of ice is 0.917 g/cm³. If a bottle with a volume of 275 mL were filled fully with water at 25º C and then frozen to -10º C, could the ice still be contained in the bottle? What would the volume of the ice be?
   No. 253 mL
   

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