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1.
An old (pre-1987) penny is nearly pure copper. If such a penny has a mass of 3.3 g, how many moles of copper atoms would be in one penny? 3.3g /63.5g = .052 moles
2.
Four nails have a total mass of 4.42 grams. How many moles of iron atoms do they contain? 4.42 g / 55.8 g = .0792 moles
.0792 moles x 6.02 x 10 23 atom/mole = 4.77 x 10 22 atoms
3.
A raindrop has a mass of 0.050 g. How many moles of water does a raindrop contain?
.050g /18g = .0028 moles molar mass H
2
O = 18 g/mol
4.
One box of Morton’s Salt contains 737 grams. How many moles of sodium chloride is this? NaCl molar mass = 23 +35.5 = 58.5 g/mol
737 g/ 58.5 g = 12.6 moles
5.
A chocolate chip cookie recipe calls for 0.050 moles of baking soda (NaHCO
3
). How many grams should the chef mass out? Molar mass NaHCO
3
= 23+ 1+ 12+ 3(16) = 74g/ mole
.050 mole x 74g/mol = 3.7g
6.
First-century Roman doctors believed that urine whitened teeth and also kept them firmly in place. As gross as that sounds, it must have worked because it was used as an active ingredient in toothpaste and mouthwash well into the 18th century. Would you believe it’s still used today? Thankfully, not in its original form! Modern dentists recognized that it was the ammonia that cleaned the teeth, and they still use that. The formula for ammonia is
NH3. How many moles are in 0.75 g of ammonia? How many molecules? How many atoms of N? of H? molar mass NH
0.75g/17g = 0.044 mole NH
3
3
= 14 +3= 17g/mol
.044 mole x 6.02 x 10 23 molecules = 2.66 x 10 22 molecules NH
3
2.66 x 10 22 molecules NH
2.66 x 10 22 molecules NH
3
x 1 N/1NH
3
x 3 H/1 NH
3
= 2.66 x 10 22 atoms N
3
= 7.98 x 10 22 atoms H
7.
Lead (II) chromate, PbCrO4, was used as a pigment in paints. How many moles of lead chromate are in 75.0 g of lead (II) chromate? How many atoms of oxygen are present?
molar mass PbCrO4 = 207.2 + 52+ 4(16)= 323.2g/mol 75g/323.2g = .232 mol
.232 mol x 6.02 x 10 23 molc/mol x 4 atoms Oxygen/1 mlc PbCrO4 =
5.59 x 10 23 atoms oxygen
8.
A rigid steel cylinder has a volume of 20.0 L. It is filled with N
2
gas to a final pressure of
20.00 atm at 27.0 o C. How many moles of N
2
gas does the cylinder contain?
VP=nRT n=VP/RT = (20L x 20 atm) /(.0821 L atm/mol k x 300K) = 16.2 moles
9.
An average person breathes in around 550 L of oxygen (O
2
) each day. How many moles is that if the oxygen is at STP? How many grams of O
2
If the O
2 that would be breathed in.
is this?
is at 25 o C and a pressure of 0.920 atm calculate the number of moles and mass
At STP 550L/22.4L = 24.5 mol O
2
24.5 mol x 32g/mol = 784g O
2
At 25 o C and .920 atm n=VP/RT (550L x .920 atm) /.0821 L atm/k mol x 298K) =
20.7 mol 20.7mol x 32g/mol = 661g of O
2
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10.
What pressure will be exerted by 0.450 mole of a gas at 25 o C if the gas is contained in a vessel with a volume of 650. ml?
VP= nRT P= nRT/V
(.450mol x.0821 l atm/k mol x 298 K)/.650 L = 16.9 atm
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