Mass & Conversions

Under mass conversions, connections between grams, moles and molecules are established. 

Isotopes

Concept: Understanding Atomic Mass & Isotopes 

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Isotopes are elements with the same number of protons, but different number of neutrons.

Concept: Calculating the average mass of an element 

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Example: Antimony has two common isotopes. If one of the isotopes 121Sb has an isotopic mass of 120.9038 amu and a natural abundance of 57.25%, what is the isotopic mass (to 4 significant figures) of the other isotope? The atomic mass of antimony is 121.8 g/mol. 

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Example: The atomic mass of an imaginary element A is 251.7 amu. If element A consists of two isotopes that have atomic masses of 250 and 253 respectively, what is the natural abundance of each isotope?

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Mass Conversions

Concept: Avogadro’s Number, Moles, Molecules & Atoms 

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The moles of a compound or element serve as the bridge between grams, molecules and atoms. 

Example: Determine the mass (in grams) found in 7.28 x 1028 nitrogen atoms.  

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Example: Determine the how many molecules of carbon dioxide, CO2, are found in 75.0 g CO2.

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Mass Conversion Calculations

Problem: If the density of water is 1.00 g/mL at 25oC calculate the number of water molecules found in 1.50 x 10^3 µL of water.

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Problem: Calculate the number of oxygen atoms found in 783.9 g in copper (II) sulfate pentahydrate, CuSO4 x 5 H2O.

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Problem: The density of the sun is 1.41 g/cm3 and its volume is 1.41 x 10^27 m3. How many hydrogen molecules are in the sun if we assume all the mass is hydrogen gas?

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Problem: A cylindrical copper wire is used for the fences around a house. The copper wire has a diameter of 0.0750 in. How many copper atoms are found in 5.160 cm piece? The density of copper is 8.96 g/cm3. ( V = pi x r^2 x h )

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