Mole Problems
6.022 x 1023 particles = 1 mole of something
6.022 x 1023 atoms = 1 mole of an element
Example: 20.456 x 1023 atoms of carbon = __?__ moles of carbon
Set up a T-chart with your starting info in the upper left hand corner:
20.456 x 1023 atoms___
1 mole______
6.022 x 1023 atoms
= 3.4 moles carbon
1. 12.589 x 1023 atoms of hydrogen = _2.09_ moles of hydrogen
2. 50.459 atoms of lithium = _8.38 x 10-23__ moles of lithium
3. 1289 atoms of barium = _2.14 x 10-21_ moles of barium
Formula Mass
Example: What is the formula mass of H2O? = (H x 2) + (O)
=
(1.00794)(2) + (15.9994)
=
18.015 grams
1. What is the formula mass of NaCl? (22.98977) + (35.453) = 58.44277 grams
2. What is the formula mass of Na2CO3? (2)(22.98977) + (12.011) + (3)(15.9994) = 105.98874 grams
3. What is the formula mass of Al2(SO4)3 = (2)(26.98154) + (3)(32.06) + (12)(15.9994) = 342.13588 grams
Percent Composition Problems
1. Calculate the percent composition of the following:
a. NH3
Formula mass = 17.03052 grams
N: 82.24%
H: 17.76%
b. C6H6
Formula mass = 78.11364 grams
C:92.26%
H: 7.74%
c. N2H4
Formula mass = 32.04516 grams
N: 87.42%
H: 12.58%
d. P2O5
Formula mass = 141.9445 grams
P: 43.64%
O: 56.36%
2. Calculate the percent composition of copper (I) sulfide. Cu2S
Formula Mass: 159.152 grams
Cu: 79.86%
S: 20.14%
3. Calculate the percent composition of sucrose, C12H22O11, and glucose, C6H12O6.
Formula Mass of C12H22O11: 342.300 grams
Formula Mass of C6H12O6: 180.15768
C: 42.10%
H: 6.48%
O: 51.52%
C: 40.01%
H: 6.71%
O: 53.28%
4. Determine the percent of water by mass in sodium carbonate decahydrate, Na2CO3 • 10H20.
Formula mass of Na2CO3 • 10H20= 286.14164 grams
H20: [(20)(1.00794) + (10)(15.9994) ] /( 286.14164) x 100
= 62.96%
5. A sample of an unknown compound with a mass of 0.2370 g is extracted from the roots of a plant.
Decomposition of the sample produces 0.09482 g of carbon, 0.1264 g of oxygen, and 0.0158 g of hydrogen.
What is the percent composition of the compound?
C: 0.09482 / 0.2370 x 100 = 40.01%
O: 0.1264 / 0.2370 x 100 = 53.33% H: 0.0158 / 0.2370 x 100 = 6.67%
6. Find the percent composition of a compound that contains 2.3 g of sodium, 1.6 g of oxygen, and 0.1 g of
hydrogen in a sample of 4 g of the compound.
Na: 2.3 / 4 x 100 = 57.5%
1.6 / 4 x 100 = 40.0%
0.1 / 4 x 100 = 2.5%
7. A sample of an unknown compound with a mass of 0.562 g has the following percent composition. 13%
carbon, 2.2% hydrogen, and 84.8% fluorine. When this compound is decomposed into its elements, what mass
of each element would be recovered?
C: (0.562)(0.13) = 0.07306 grams
H: (0.562)(.022) = 0.012364 grams
F: (0.562)(.848) = 0.476576 grams