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14GramsToMolesToMolecules

# 14GramsToMolesToMolecules - Chemistry 2A In-Class Worksheet...

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Chemistry 2A Dr. Freitas In-Class Worksheet Topics: Grams-to-Moles-to-Molecules problems Information: Pages 1-4 are examples. Page 5 is a set of Grams-to-Moles-to-Molecules problems Avogadro's number: 6.02 x 10 23 objects = 1 mole; the objects are molecules, ions or atoms. Once you have tried the problems by category, you need to SCRAMBLE them and see if you can still do the problems correctly! That is how they will appear on the exam! Helpful HINTS : In these problems look for two things: 1) from what unit to what unit? and 2) does the object stay the same, or does the object change? (e.g. molecules to molecules is when the object stays the same, molecules to atoms is when the object changes). EXAMPLES Involving Atoms : When dealing with atoms by themselves , for example, atoms of a noble gas (Ar, Xe, Kr) or atoms in a metal (Cu, Zn, Na) and not as part of a molecule or polyatomic ion, then we do not convert to ions or molecules. Example1 How many moles of Ar are there in a sample of gas that contains 2.84 x 10 25 Ar atoms? Overall: We are starting with units of Ar atoms and our answer should be in units of moles of Ar. The object stays the same. Since Ar is by itself, we cannot convert to a larger unit at the molecular level. We go straight to moles using Avogadro's number. Step 1: Convert atoms to moles. 2.84 x 10 25 Ar atoms x 1 mole Ar = 6.02 x 10 23 Ar atoms Example 2 If we had a sample of 32.5 grams of copper, how many copper atoms would be in the sample? Overall: We are going from units of grams of Cu to individual Cu atoms . Large to small. We always convert from Large to small going through MOLES. The object stays the same. Step 1: Find the Molar Mass (units of gram/mole) and convert from grams to moles Step 2: Use Avogadro's number. The object in this problem is atoms, because a metal Cu is thought of in terms of individual atoms rather than in a molecule. Convert from moles to atoms. 32.5 g Cu x 1 mole Cu x 6.02 x 10 23 Cu atoms = 3.08 x 10 23 63.55 g Cu 1 mole Cu Example 3 If we put 5.76 x 10 28 atoms of Zn on a gram balance, what would the balance read? Overall: Again, Zn is an isolated atom, not part of a molecule or polyatomic ion so we don't have to convert through ions or molecules. The object stays the same. We are going from units of individual atoms to grams : small to large. Step 1: Convert atoms to moles, using Avogadro's number. The object is atoms, because Zn is made of individual atoms. Step 2: Convert moles to grams using the molar mass.

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14GramsToMolesToMolecules - Chemistry 2A In-Class Worksheet...

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