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Molarity & Solutions

Prescription drugs in the correct concentration make you better.In higher concentration they can kill you.

Chemists need to make solutions that have precise concentrations

Pesticides must be in proper concentrations.

Food additives must be in correct concentrations.

A driver is legally impaired at 0.08 mg/mL blood alcohol content.

Chemists control the concentration of chemicals using the concepts we will develop in this unit.

Concentration

Concentration-the amount of solute that is dissolved into a solvent

Concentrated-when a solution has a high concentration of solute

Dilute-when a solution has a low concentration of solute

Concentration…

Concentration can be described in terms of “weak” or “strong”, “concentrated” or “dilute”, but these are both relative terms and can be subjective.

Molarity

Molarity is one way to measure concentration; it is the number of moles of solute per liter of solution.

The unit of molarity is mol/L, which can be abbreviated with an uppercase M.

The concentration of a solution tells you how much solute that is dissolved in a given amount of solvent (water).

The molarity is the concentration of a solution.

Molarity = MolesLiter

1. 2.255 moles of NaCl is dissolved in 4.0 L of water, calculate the molarity.

Molarity =

The concentration of a solution tells you how much solute that is dissolved in a given amount of solute (water).



1. 2.255 moles of NaCl is dissolved in 4.0 L of water, calculate the molarity.

Molarity = 2.255 moles4.0 L

The concentration of a solution tells you how much solute that is dissolved in a given amount of solute (water).



1. 2.255 moles of NaCl is dissolved in 4.0 L of water, calculate the molarity.Molarity = 2.255 moles = 0.56 moles = 0.56 M(Molar)

4.0 L Liter

2. 5.00 g KOH is dissolved in 250. mL of water, calculate the molarity.

Molarity =


Molarity = 5.00 g 1


K = 39.0 (mass off periodic table)O = 16.0H = 1.0 56.0 g/mol

Molarity = 5.00 g x 1 mole 1 56.0 g




0.250 L




0.250 L

= 0.356 M

3. How many moles are there in 205. mL of a 0.172 M solution?


0.172 moles 1 L


0.172 moles x 0.205 L 1 L 1


0.172 moles x 0.205 L = 0.0353 moles 1 L 1

4. How many grams NaCl are there in 250.0 mL of 0.500 M solution?


0.250 L

1


0.250 L x 0.500 moles

1 1 L


0.250 L x 0.500 moles x 58.5 g

1 1 L 1 mole


0.250 L x 0.500 moles x 58.5 g = 7.31 g

1 1 L 1 mole

5. How many grams of NaCl must be used to prepare a 100.0 mL of 0.250 M solution.


0.1000 L 1


0.1000 L x 0.250 moles 1 1 L


0.1000 L x 0.250 moles x 58.5 g 1 1 L 1 mole


0.1000 L x 0.250 moles x 58.5 g = 1.46 g 1 1 L 1 mole

6. How many milliliters of a 0.200 M solution of CoCl2 will contain 25.0 g?


25.0 g 1


25.0 g x 1 mole 1 129.9 g


25.0 g x 1 mole x 1 L 1 129.9 g 0.200 mol


25.0 g x 1 mole x 1 L x 1000 mL 1 129.9 g 0.200 mol 1 L


25.0 g x 1 mole x 1 L x 1000 mL = 962 mL 1 129.9 g 0.200 mol 1 L

7. Describe how you would prepare 100.0 mL of a 0.200 M solution of CoCl2.

0.1000 L x 0.200 moles x 129.9 g = 2.60 g1 L 1 mole

Weigh 2.60 g of CoCl2

Dissolve in water

Transfer to a 100 mL volumetric flask and fill to the line