Solutions and Molarity

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Solutions and Molarity

• 4-20-09

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What is a Solution?

• A Solution is a homogeneous mixture composed of
  two or more substances.
• Example Salt Water
• (Fig 13-2, pg 469)
• NaCl(s) ? Na(aq) Cl(aq)

H2O
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Solutions Consist of Two Things

• Solvent is the substance in which the solute
  dissolves to make the solution. (Example Water)
• Solute is the substance dissolved in a solution.
  (Example Salt)
• Solutions are often called Aqueous. (A solution
  in which the solvent is H2O.)

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Expressing Concentrations

• The quantity of solute in a specific quantity of
  solvent is a measurement of the Concentration of
  the solution.
• Concentration is measured in units of Molarity.
• Molarity is the ratio of solute amount (in moles)
  to solution volume (in liters).

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Molarity

• Molarity
• M
• What is the Molarity of a solution that has a
  volume of 1.5L and contains 3.5 mols of NaCl?

moles of solute liters of solution
mol L
3.5 mols 1.5 L
2.33 M
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Sample Problem 1

• What is the Molarity of a solution that has a
  volume of 2.5L and contains 4.3 mols of NaCl?
• But what if the question involved mL instead of L?

4.3 mols 2.5 L
1.72 M
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Example (mL to L)

• What is the Molarity of a solution that has a
  volume of 400mL and contains 5.2 mols of NaCl?
• Now you try one!

5.2 mols 400 mL
1000 mL 1 L
13 M
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Sample Problem 2

• What is the Molarity of a solution that has a
  volume of 250mL and contains 3.8 mols of NaCl?
• But what if the question involved grams instead
  of mols?

3.8 mols 250 mL
1000 mL 1 L
15.2 M
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Example (grams to mols)

• What is the Molarity of a solution that has a
  volume of 1.6L and contains 60 grams of NaCl?
• Now you try one!

60 g NaCl 1.6 L
1 mol NaCl 58.45 g NaCl
0.64 M
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Sample Problem 3

• What is the Molarity of a solution that has a
  volume of 2.3L and contains 95 grams of NaCl?
• But what if you were just given grams and mL in
  the question?

95 g NaCl 2.3 L
1 mol NaCl 58.45 g NaCl
0.71 M
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Example (mL to L grams to mols)

• What is the Molarity of a solution that has a
  volume of 600 mL and contains 75 grams of NaCl?
• Now you try one!

1000 mL 1 L
75 g NaCl 600 mL
1 mol NaCl 58.45 g NaCl
2.14 M
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Sample Problem 4

• What is the Molarity of a solution that has a
  volume of 900 mL and contains 100 grams of NaCl?
• Now what if I wanted to
• make my own Molarity solution?

1000 mL 1 L
100 g NaCl 900 mL
1 mol NaCl 58.45 g NaCl
1.90 M
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Example (Solute from Molarity)

• How many grams of NaCl should be added to make a
  100mL flask of a 3.5 M solution?
• Now you try one!

mol NaCl
1 L 1000 mL
100mL
3.5 1 L
58.45 g NaCl 1 mol NaCl
20.46 g of NaCl
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Sample Problem 5

• How many grams of NaCl should be added to make a
  500mL flask of a 5.4 M solution?
• Can I dilute a Molarity
• soln to make a new one?

mol NaCl
1 L 1000 mL
500mL
5.4 1 L
58.45 g NaCl 1 mol NaCl
157.82 g of NaCl
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Example of Dilutions

• You obtain 50 mL of a 4 M solution of NaCl. How
  many mL are in the final volume of a 2 M solution
  of NaCl?

M1V1 M2V2
(2M)
(4M)
(50mL)

(?mL)
(200)

(2)
(?mL)
Divide by 2 on both sides
mL

100
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Sample Problem 6

• You obtain 75 mL of a 5 M solution of NaCl. How
  many mL are in the final volume of a 4 M solution
  of NaCl?

M1V1 M2V2
(4M)
(5M)
(75mL)

(?mL)
(375)

(4)
(?mL)
Divide by 4 on both sides
mL

93.75