CHM 2045C Learning Tools

   Mole Calculations 

The answers and work are at the bottom of this exercise. I recommend that you attempt all of the questions before checking the answers. Remember to check your work for the appropriate number of significant figures.
1. Calculate the number of moles of chlorine atoms found in 3.50 moles of aluminum chlorate.
2. Calculate the mass of bromine in 2.913 g chromium(III) bromide.
3. 9.0 x 1021 molecules of dinitrogen tetraoxide contains how many atoms of oxygen?
4. Calculate the mass of calcium chloride dihydrate that contains 25 g of water.
5. Calculate the moles of tin(IV) sulfite that contains 100. g of sulfur.
6. Calculate the atoms of nitrogen in 3.25 g of manganese(III) nitrate.
7. Calculate the number of formula units of rubidium phosphide that contain 12.0 g of rubidium.
8. Determine the moles of zinc hydroxide that contains 8.34 x 1024 hydroxide ions.
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Answers
1.            10.5 moles Cl
3.50 moles *(3 moles Cl)/(1 mol Al(ClO3)3) = 10.5 moles (3 sig fig in 3.50 moles)
2.            2.394 g Br
2.913 g CrBr3 * (239.73 g Br)/(291.73 g CrBr3) = 2.394 g Br (4 sig fig in 2.913 g)
A second approach:
2.913 g CrBr3 *(1mol CrBr3/291.73 g CrBr3)*(3 mol Br/1mol CrBr3)*(79.91g Br/1mol Br)
3.           3.6 x 1022 atoms O
9.0 x 1021 molecules N2O4 *(4 atoms O/1 molecule N2O4) = 3.6 x 1022 atoms O (2 sig fig in 9.0)
4.          1.0 x 102 g CaCl2 * 2H2O
The compound is CaCl2 * 2H2O.  It contains 2 moles of water per mole of compound (cpd).
25 g H2O * (147.02g cpd)/(2 * 18.02 g H2O) = 1.0 x 102 g cpd (2 sig fig in 25 g)
A second approach:
25g H2O *(1mol H2O/ 18.02 g)*(1 mol cpd/2 mol H2O)*(147.02 g cpd/1mol cpd)
5.            1.56 mol Sn(SO3)2
100. g S *(1 mol S/32.06 g S)*(1 mol Sn(SO3)2/2 mol S) = 1.56 mol Sn(SO3)2 (3 sig fig is 100. g)
6.            2.44 x 1022 atoms N
3.25 g Mn(NO3)3 * (1mol cpd/240.97 g cpd)*(3 mol N/1 mol cpd)*(6.022 x 1023 atoms N/1 mol N) =
2.44 x 1022 atoms N (3 sig fig in 3.25 g)
7.         2.82 x 1022 formula units Rb3P
12.0 g Rb *(1 mol Rb/85.47 g Rb)*(1mol Rb3P/3 mol Rb)*(6.022 x 1023 f.u. Rb3P/1 mol Rb3P) =
2.82 x 1022 formula units Rb3P (3 sig fig in 12.0 g)
8.          6.92 mol Zn(OH)2
8.34 x 1024 OH- ions* (1 mol ions/6.022x1023 ions)*(1 mol Zn(OH)2/2 mol OH- ions) = 6.92 mol Zn(OH)2
(3 sig fig in 8.34 x 1024)
 
Last modified September 26, 1998                  Top        For more information, contact:  ksanchez@fccj.org