Assignment 026


Question 1
Calculate the mass of 2.18 moles of chromium atoms.


Question 2
How many moles of sulfur atoms are in 3.17 g of sulfur atoms?


Question 3
How many silver atoms are in 6.89 g of silver atoms?


Question 4
Calculate the mass of 8.64x1023 silver atoms.


ANSWERS


Question 1 (00000001A0601402, Variation No. 70): 113 g

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You need to refer to a periodic table and look up the atomic weight of chromium. You will find it to be 52.00 (rounded to four significant figures). This means that:

The molar mass of chromium is 52.00 g/mol.
One mole of chromium atoms has a mass of 52.00 g

Therefore: n moles of chromium atoms will have a mass of (n TIMES 52.00) g.
OR: 2.18 moles of chromium atoms will have a mass of (2.18 TIMES 52.00) g, which is equal to 113 g.

Here's how it's done using dimensional analysis:

2.18 mol chromium atoms

 

52.00 g chromium

 

 

 

x


=

113 g chromium

 

 

1 mol chromium atoms

 

 



NOTES:

1. "mol" is the official abbreviation for "mole" (or "moles").

2. The number of significant figures in our molar mass should, if possible, exceed that in the measured mass. Since there are 3 significant figures in 2.18 g; it is sufficient to round off our molar mass to four significant figures (52.00 g/mol).


Question 2 (00000001A0601403, Variation No. 53): 0.0988 mol sulfur atoms

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You need to refer to a periodic table and look up the atomic weight of sulfur. You will find it to be 32.07 (rounded to four significant figures). This means that:

The molar mass of sulfur is 32.07 g/mol.
One mole of sulfur atoms has a mass of 32.07 g

Therefore: n moles of sulfur atoms will have a mass of (n TIMES 32.07) g.
The problem says: 3.17 g EQUALS (n TIMES 32.07) g; so we solve for n by dividing 3.17 by the molar mass (32.07).

Here's how it's done using dimensional analysis:

3.17 g sulfur atoms

 

1 mol sulfur atoms

 

 

 

x


=

0.0988 mol sulfur atoms

 

 

32.07 g sulfur atoms

 

 

 


Question 3 (00000001A0601404, Variation No. 57): 3.84x1022 silver atoms

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You need to refer to a periodic table and look up the atomic weight of silver. You will find it to be 107.9 (rounded to four significant figures). This means that:

The molar mass of silver is 107.9 g/mol.
One mole of silver atoms has a mass of 107.9 g

Therefore: n moles of silver atoms will have a mass of (n TIMES 107.9) g.
The problem says: 6.89 g EQUALS (n TIMES 107.9) g; so we solve for n by dividing 6.89 by the molar mass (107.9). This (n) gives us the mole count. To get the individual atom count, we multiply n by Avogadro's number.

Here's how it's done using dimensional analysis. First, we calculate the number of moles:

6.89 g silver atoms

 

1 mol silver atoms

 

 

 

x


=

0.06386 mol silver atoms

 

 

107.9 g silver atoms

 

 



THEN, we convert mole count to individual atom count:

0.06386 mol silver atoms

 

6.02x1023 silver atoms

 

 

 

x


=

3.84x1022 silver atoms

 

 

1 mol silver atoms

 

 



Here's a more direct approach, since 1 mole of atoms is 6.02x1023 atoms:

6.89 g silver atoms

 

6.02x1023 silver atoms

 

 

 

x


=

3.84x1022 silver atoms

 

 

107.9 g silver atoms

 

 


Question 4 (00000001A0601405, Variation No. 28): 1.55x102 g silver atoms

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You need to refer to a periodic table and look up the atomic weight of silver. You will find it to be 107.9 (rounded to four significant figures). This means that:

The molar mass of silver is 107.9 g/mol.
One mole of silver atoms has a mass of 107.9 g

Therefore: n moles of silver atoms will have a mass of (n TIMES 107.9) g. We can calculate (n) by simply dividing the individual atom count by Avogadro's number. Then multiply (n) by the molar mass to get the answer.

Here's how it's done using dimensional analysis. First, we convert individual atom count to mole count:

8.64x1023 silver atoms

 

1 mol silver atoms

 

 

 

x


=

1.435 mol silver atoms

 

 

6.02x1023 silver atoms

 

 



THEN, we convert moles to grams:

1.435 mol silver atoms

 

107.9 g silver atoms

 

 

 

x


=

155 g silver atoms

 

 

1 mol silver atoms

 

 



Here's a more direct approach, since 1 mole of atoms is 6.02x1023 atoms:

8.64x1023 silver atoms

 

107.9 g silver atoms

 

 

 

x


=

155 g silver atoms

 

 

6.02x1023 silver atoms